Roles Can Be A Bummer
Making actions/options not available to people can force people out of the magic circle pretty quickly. Sure, roles can help people feel special, but it can go overboard. This does not mean that the rogue shouldn't be stealthy, or that the cleric can't be tank-y, but not being able to use or do something because of an arbitrary class restriction breaks the circle, which means less fun for everyone.
Sometimes, multiple people actually can do the same thing, and that should happen sometimes. If every task you throw at the player characters cannot be accomplished except for 1 person's specialization, it may be a harder adventure than these players (as a group) are fit for. Not that there can't be a situation where you need a particular player to do a something, but that it shouldn't happen all the time.
Perhaps It is a Matter of Perception
So maybe this player is simply just does not imagine the same thing you do. You can try to reinforce what you envision by describing what is going on. Is she/he making informed choices? Does she/he realize what the gravity of the situation, or how her/his character has certain strengths?
Does this player realize the cleric is running around in thick armor, while that player's character is running around in normal clothes? For example, if I think the cleric is running around in normal clothes, and goes into combat and comes out unscathed, I could run in my normal clothes, and also come out unscathed.
If this is the problem, you simply need to be more descriptive! Describe how the cleric is wearing armor, or how the rogue is especially cat-like, and how this player's character is not.
For Games Which Depend On Roles
It may be time to have your other players step up. Have the cleric yell at the other character to stay back, or that the cleric "has this." Alternatively, the cleric may ask "PEASE SUMMON THIS TO HELP ME!" as he charges in.
If the rogue attempts to sneak around, the rogue can give specific instructions; "Can you make a distraction over there?" "Wait here, and if I'm not back in 15 minutes, go get the others to rescue me." Or even: "this looks super dangerous. I don't think you'll make it. We don't want to get ourselves killed, yeah?"
In short, give him tasks, or have the other players give him tasks to help with things. Talking is a free action in combat; use it. This is especially good for players who may be experiencing some mental handicap; it gives them something concrete to act on. It allows them to contribute (which feels great and is fun) without going through negative experiences.
Finally, a player or the DM can specifically highlight when certain actions will require a specific specialization, and who has that training. If it becomes obvious that a task is dangerous and requires training, then most people leave it to the person with the training. The DM can further forbid people without specialization from trying, stating that it's obviously too hard. This is more "hand-holdy" than some people like it, but sometimes people just need their hands held.
Talk About It
Talk with the player about the roles. What is her/his character good at? What should she/he focus on? These other characters have something special about them, what is she/he special at?
Talk about how their character is so good at magic/whatever else, and how they should try to make their magic/whatever else be the solution for the current situation. (After all, we're solving these situations using our strengths, use your strengths to help solve it!)
Tough Love
Finally, you may have to resort to letting the consequences of this player's action happen. Let the events, despite other character's best efforts, happen. Talk about how that character was not focusing on what they were good at, or how their play-style does not match up well with the class they chose. Make a new character that does okay at everything, such as most games' version of a "bard." Note: this does not mean make a character who is rule-breaking and superior to all the others, but one that stands a decent chance at performing many things, and isn't a large risk to the success of most tasks.
Is the problem laziness or lack of skill?
This problem can be approached in either of two ways: treat the symptoms or treat the disease. I would attempt both of them, in that order.
Treat the symptoms: aka How do I Rogue?
As a core class, the Rogue is in the SRD. Given your party's use of digital means this should not be a problem to use. Character sheets do not have space to write the rules of a class feature, but here they are listed for your use. Ask of your players that need to that they write down their class features, feats, skills and magic items and what they do. This way they have the rules on hand at all times. Make them aware what these things can do for their character and the party.
Even then, there's something of a problem. The party consists of one melee guy, two casters and the Rogue. This means that the Paladin is going to be bearing the brunt of the melee attacks, and can easily be swarmed. While there's a good chance to flank a guy fighting the Paladin, the moment that the attacker drops the Rogue is out in the open. This is NOT a good thing for a Rogue with their light armor and d6 hit die. Make the player aware of this, preferable before combat with a large group.
The cure: how lazy is the player?
The real question: is the player really lazy? Sure, they might have joked about this but it can mean that they are insecure. The biggest clowns are the saddest clowns, after all. Ask them, in private without the other players knowing, if they would like help. Do not make it a confrontation or an accusation: make it an offer and a gesture of good will. Perhaps they do not know how to play a Rogue. Hell, I know that I could not Rogue to save my life. Players who are insecure are not the best teachers either: their mistakes will carry over to the others. Offer to take on the mantle of the sorceror's mentor (try saying that three times quickly) so they can focus on their Rogue.
On the other hand though, if they actually ARE being lazy, tell them that they need to put some time in the books to actually play. You (and few people do) do not like to be bogged down by the player unwilling to learn the rules, as opposed to one who just does not know them. Tell them what you expect of them and when you do so. When this time comes, ask if they managed to do the thing you asked. If they did, great! If not, ask if they need more time or some help (NEVER ask "why not?": this helps nobody).
If they are consistently unable to do so or learn the rules, you should ask them (in private!) if they want to play D&D, or if they would rather be doing something else. If this is the case, shake hands and go your own ways.
Prevention!
You want to start immediately when everyone's at the table. That's fine, but it needs preperation. Ask your players to send in their character sheets for you to check if they are what you expect (do not call it "did it right": this makes you look like a jerk). If everything is fine, great! If not, send feedback. If characters are written digitally ask for prints: if they're filled out by hand ask for pictures (you have WhatsApp, so you likely have phones with cameras). Also ask for the "cheat sheets" that you would like to see of some people. This is not a lot of work: just copypaste from the D20 SRD.
Final advice: Relax!
These events seemed to have rather vexed you. Take a deep breath, have a talk with your players and go and have fun the next time you gather for a game. If the DM is not having fun, it will be difficult for the players to have fun.
Best Answer
This needs to be dealt with out of game
Although other answers are advocating punishing his behaviour, of what I've seen of various other problem-players Q&As, that rarely works, since the problem is actually a real-world problem, not a game problem, and that is that this player's behaviour is negatively impacting the fun of the other players, and you, the DM (you count as a player for the purposes of "are you having fun?").
Although there is no right way to play D&D, it's fair to say that he's playing it the wrong way; ultimately, D&D is a game about team work, and he's actively refusing to be part of the team, even when the other characters and players are asking him to join in. The equivalent is a group of friends meeting up to watch a film together, and one person is playing on a handheld gaming device with the volume turned all the way up, or having a loud conversation on their phone next to everyone else, just because that's what they want to do, to the detriment of everyone else's fun.
What I'd recommend first is talking to the player separately, away from the group. Tell him about how his character's actions (which is actually your player's behaviour, since characters can't do anything by themselves; see also "My Guy Syndrome") is negatively impacting the fun of the other players, including you (remember, you are a player too), specifically the constant seductions and refusing to join in with combat that affects the whole team. He will either claim that he'll try to change, or he'll refuse to see the problem.
The problem here is a social one, so try to keep the focus away from "that's not how you play D&D", or "I don't like your character", but more on the real problem: "Your choices are making me/us uncomfortable", and "I'm/we're not enjoying the game as much as we could because of this" (if you use "us" or "we're", it might be worth confirming this with the others first away from Will so that you aren't putting words in their mouths).
If this fails to work (meaning, even after the conversation, the behaviour persists, especially if he refuses to see the problem), then what you need is a session 0 (if you had one already, you can have another). This is where the group gets together to discuss the game, specifically the tone of the game (i.e. not trying to seduce everything, which often makes a lot of people uncomfortable; there are other Q&A's on this site about players doing this and upsetting their fellow players in the process) and what you each expect from each other (i.e. actually help out in combat, don't just sit in a wagon and play with your dog).
If Will's behaviour still doesn't improve, this is where you need to discuss with the group (minus Will) about kicking him out. At this point, you've at least shown that you've tried to encourage him to improve, but he's just not respecting the rest of you. Hopefully they will see at this point that Will has brought this on himself, and even if you push your decision through (i.e. if others are saying "give him another chance", but you're like "No, he's had enough chances and I don't think he's going to improve"), then at least the onus of responsibility is on him now.
And as harsh as this may sound, with regards to hurting his self-esteem, some people will only learn the hard way, and you cannot pander to him indefinitely without teaching him that his actions have no consequences, and he therefore doesn't ever need to improve his behaviour or learn to respect others. You never know, it might take being kicked out to make him realise that he needs to play nice, and once he's learned his lesson, if he does actually want to play, he could even be invited back in on the proviso that he doesn't behave how he did before?