I would say that there is an easier way to solve this than having a "sitdown".
My advice would be to play along and let him take whatever character he wants to play. Then have this character have to deal with the way that the rest of the game world views his behavior.
For instance, if he walks into a 7-11 with his guns on hooping and hollering, have the clerk call the cops and throw him in jail.

This would happen in the real world wouldn' t it?
If he says something in character, have it come out in the game.
You can also play it out in the game to resolve it. For example, perhaps the players lose a contract for a mission because the client thinks that the wacky truck driver would kill his kidnapped daughter instead of the kidnappers. Have the players help to settle it in the game.
I have played in groups with people like this before and this approach worked. I don't like to tell people how or what to play, but they have to deal with the consequences of thier choices in the game regardless.
There shouldn't be "normal" character. Standard is a better word perhaps. I think that any player who enjoys playing his character should do so, but remembering that they have to deal with how thier player clashes with the world, Just like me and my plaid pants!
You should have seen how Dave Arneson reacted to my Halfling Barbarian!! I was a frontline fighter and man I was kicking butt! Sorta breaking the hobbitt mold eh.

Maybe I am known as one of these types of players as well..
Dustin