When a player is hogging the limelight like this, the way to deal with the situation is to stop encouraging them. The player is getting their fun by having everyone's attention focused on them (see this question for a similar situation). (This isn't a bad thing, by the way! It just means you have to make sure that the rest of the group gets their fun, too.)
Don't Reward Indecisiveness
Right now, it sounds like the entire group is (unintentionally) rewarding the player by watching when he's roleplaying, letting him stall, and otherwise paying attention to him. So the first thing you need to do is to stop paying attention to him. That sounds harsh, but doesn't have to be.
If the player refuses to make a decision in-character, the DM should simply say, "Okay. While you're thinking about that, I'm going to switch scenes to the other characters for a bit. Let me know when you've made a decision." Then the DM swaps focus to the rest of the group and ignores the player (except to answer the occasional reasonable question, such as a request to make a knowledge roll to help make a decision). This is going to be difficult for the DM, because the player will immediately do everything they can to get the attention back - asking lots of questions and demanding immediate answers, trying to RP more stalling, etc - but it's up to the DM to repeat, "I've given you all the information your character has. Let me know when you've made a decision." Then the DM must return to ignoring the player until they take action.
Similarly, the other players can help by not letting the player sidetrack or delay discussions. I've DMed and played in a number of games where one person (in- or out-of-character) couldn't make up their minds about what to do. If the rest of the group couldn't convince them to take an action in a couple of minutes of discussion, the other players would just forge on ahead, telling the character IC, "Catch up when you're ready, or stay here and guard the horses, it's up to you." Then either the player would say, "Screw it," and go along with the group, or the player came up with a reason to stay behind and sat out the following scene(s).
In cases of slow, drawn-out RP, it's perfectly reasonable for the NPCs with whom the character is speaking, to get impatient and interrupt the PC, talk over him, make assumptions, or otherwise do any of the things actual humans do when they get impatient. So if the player is giving a drawn-out William Shatner speech about "My... disease... (cough) is getting... (cough) a lot... (cough) worse... (cough) and I need...", most NPCs are likely to interrupt at this point with, "Yeah, yeah, you need the local cleric. Two streets over, third building on your right. Have a nice day!" Then the NPC will back away quickly and refuse to speak further (because who wants to spend any time around someone who's obviously sick and coughing up gross phlegm?). Note that the NPCs don't have to - and shouldn't necessarily - answer the question that the PC was actually going to ask. They're impatient and grossed out; they're going to make an assumption and skedaddle before the PC can correct them. The player will quickly figure out that if he doesn't get to the point, NPCs won't stick around long enough for him to talk to.
Do Reward Fast Movement
Rewarding the player when he acts quickly, whether by making a decision, roleplaying smartly, or otherwise playing in a more group-friendly way, will help encourage him to do those things more. For example, have NPCs be more helpful if he gets to the point quickly, or compliment him on his swift decision-making if he doesn't dawdle.
It's going to take a while - and a lot of work on the entire group's part - to encourage the player not to hold up the game for everyone. The player may respond to initial attempts to speed things along by being louder, or going even slower than usual. Ignore him, don't give him any attention, and don't let him slow down your game, and he'll eventually figure out that being a valued party member is much more rewarding than being the guy everyone is ignoring.
I call this "Open World" versus "Story Arc." Obviously both styles are fine but I've seen this problem as well. The sides get a little tired of each other.
If you want to bring your wanderers into the story I suggest you tie some plot points to their characters' history. Or rather, add their history to the plot points.
Examples are to add in mid-crawl bosses that insulted/betrayed them earlier. Or scour their backstories for ideas. Did they know their uncle was a necromancer? That he stole your grandfather's shield from your father? That sort of thing.
The other way I handle this is by using the open-world players to advance the story. I've always felt these folks need to know the whole world is lush and interactive.
So, if they drop the quest to find the dragon's lair to get into the slave trade they'll naturally meet someone that begs for freedom in exchange for showing them a vast treasure. Guess where that treasure is?
Look, people are of course going to get wise to this sort of thing but we all have a tendency to like stories that tie together. Some folks just don't want to follow what feels like a paved road.
Best Answer
There are several potential solutions...
1) talk with him about that behavior.
1a) bounce him out of the group if he won't stop.
2) embrace the in-character play, and quit trying to impose a story.
3) place a few stories that highlight his fixations, and encourage others to "return the favor" ... in hopes he sees how distorted it might be.
I'd honestly suggest trying #2. It may seem counter-intuitive, but the player is in fact not far out of line for a FATE system game. FATE supports strong player participation in plot creation, both by letting the GM know what buttons they want pushed (via aspects), and by truth creation and declarations.