To make your question short, and to see if I understood it correctly, we're talking about a player who made his character a certain one and roleplays it entirely different. You added that you think that it comes from inexperience, and that he created this character after you said "no" to some "freak-character"-ideas. You want to help him roleplay the character he created.
As I see it, this problem is made from two smaller ones. The first is that he doesn't see his character as interesting because the character "is normal and normal is boring". The second is that you wanna help him understand why the way he plays the character does not fit the story-world of your game.
Helping him understand that "normal is not boring
This is the more important problem, as it stands in the basis of the entire problem. If he'll see that normal characters can be interesting his "anti-persona" will perish and he'll roleplay a normal character and not a freak one. The main trick here is to show him that normal characters are not entirely normal, i.e. "no person is like the others". In order for that to work, we need to give the character depth.
The easiest way to give depth to a character is through internal conflicts. Having goals and all is nice, but without something that blocks oneself from achieving them it is far less interesting. First thing to do is to go over his character's background and see if he implemented there an internal conflict for his character. If so, show it to him and talk with him about it. If no, sit with him and help him to come with one. The internal conflict doesn't have to be extravagant, but it needs to be there. An example one might be that he loves Vincent's sister but secretly hates Vincent himself, or another like Loves the sister but thinks that he's not good enough for there. I'll take the second one as an example for this section.
The conflict gives us a few things, a few added benefits. It gives the character 2 conflicting goals: "Get the sister and prove that I'm worthy". Now, with those two we also get a kind of an achieving-plan: "If I'll show her that I'm worthy, by getting something amazing done, she'll want me and I'll be able to get her". More than that, the character gets the knowledge that each advancement in order to achieve one goal will drive the other one to the far end.
But the first conflict is even more interesting. The character here has the knowledge that he needs the brother in order to save his lover, but he just can't stand being near the brother. He'll drive the mission onward for two reasons but he'll have doubts about his lover- if he'll marry her he'll be stuck with this brother of hers.
To make long story short, simple conflicts can show the player that even normal characters are interesting and unique. When combined with goals they force the character to take certain steps along the roads, to commit certain actions along the way, that he won't want to do but will make him doubt himself and question himself and see that his problem are far more interesting than those of every freak that he'll encounter.
Another nice way to help him see the importance of conflict is through showing him and analyzing with him certain protagonists that are normal people, from the stories and movies and series (of any form)that he likes. He'll see quite quickly that the conflicts make them interesting.
But he may say that it is not enough. For that there are a few more literary tools that might help him see why normal people are interesting. The first one is having flaws (internal or external) and the second one is using "The Ghost".
Flawed characters are characters that just like normal people aren't perfect. Those flaws can be internal (self-doubts, for example, or a mild paranoia) or they can be external (they're look frightens ordinary people, for once, or a missing hand for the other). The idea is that the character has to deal with the flaw, and one day to find the strength to overcome it. The fight for the overcoming act makes the character far more interesting. A nice example of that can be seen in The Rain Man, where he learns at the end that he can count on strangers/"dumb" persons like he's brother. Another nice example can be seen in the story of The Ugly Duckling who although looking terrible learned to acknowledge himself and to accept the way he looks, to accept his difference.
"The Ghost" is an event from the past that just like a ghost haunts the character to this day. Again, trying to cope with it is what builds a deep character. One example for this can be seen in the movie Inception, where we literally have a ghost- Cob's wife. Another example for this can be seen in the movie Casablanca, where he has to deal with his broken relationship with Ilsa. This Ghost is far more interesting as the originator of the Ghost actually comes back to his life. In Frozen we see another kind of a Ghost- the act that one feels guilty about. Elsa actually killed her sister.
All of these techniques are there for one reason- to make regular people interesting, to give depth to the characters, to make them human beings with goals and drives and psychology.
Helping him see that his character doesn't fit the world
After he understands that he doesn't have to be a freak in order to be interesting, he will be far more understandable about playing a character that fits the world. Then, try to explain to him as calmly as you can what it is in the way he played his character that doesn’t fit the world.
Explain to him that the characters are in a world where being a freak is bad, where achieving one's goals is the ideal. Each and every one for himself, as the saying goes. Give him examples from the way he played his character and analyze with him, in a one-on-one conversation where his way of acting came from. Use the background he created to illustrate to him where your problem comes from.
Then ask him what problems he has with his character, and together try to find a solution. Maybe let him be just a little bit freakish. Maybe he needs to just create a different character. This is basically between you and him. After that show the updated character to the group and get their approval.
When combining those two, you'll get a player who his far more willing to both play the character while also seeing the problems with the way he played his character before.
Combining the two solutions
When combining the two solutions you get a better player, who understands for the future also how to create regular characters that are not freaks yet far more interesting than those freaks will ever be able to be. Furthermore, you get a player who is willing to play his character as written while still making the character fit into the world. Hope any of these helped you.
Best Answer
1. Carry pre-generated characters.
2. Allow character changes after-the-fact.
Pregenerated characters can save you a lot of time. It's not the ideal solution for immersive role-playing, but it'll save you what sounds like hours. Then, between sessions, allow the new player to build a character of similar level, now that they've got a taste for what they like or don't. I've got a stack of about 30 pregens[1] that I keep in my GM binder, so a new player can usually find something pretty-close to their idea and get rolling.
After the session the player can either keep the pregen, make modifications to it and keep playing it (see below), or make a new character from scratch now that they've some ideas. The pregen thus discarded could be worked into the narrative as an NPC or discarded as you see fit. (With thanks to @Cronax for the suggestion.)
Many GMs allow players to make modifications to characters well after character creation. In Adventurers League it's everything down to race, class, and ability scores until level 4[2]. Others will make reasonable allowances for changing anything that hasn't factored majorly into gameplay. This question for helping new players make characters, and its answers, contains good guidance in this vein. I, personally, will allow pretty-major rebuilds at any level; my 5e table isn't much prone to "abusing" mechanics.[3]
The purpose of this is to take the weight off of the many decisions inherent in the character creation system. The player doesn't have to worry that a poor choice of skill may not synergize with the feat they'll take at level 8 or if they've the right spells in the book.
1 - Some come from the WotC site, some from the defunct Wizards Forums. Others from "solitaire" play--rolling up characters for fun--and others were specifically built to provide character continuity from previous campaigns/systems.
2 - see page seven of the Adventurer's League Player's Guide, "Character Rebuilding"
3 - In real life I play a character in a system where it's a very hard and long process to make these sorts of changes. I don't find it fun enforcing that constraint on game-players.