This is difficult, and I'm guessing you've tried most of the simple solutions. Here are some ideas that may help:
* Have everyone work together to create characters for the game. If you're starting beyond first level (or the equivalent in another gaming system), tell them that they're creating PCs that have worked together in the past, and they're to come up with the group of PCs together. If you're starting at first level, require every PC to have strong connections to at least two other PCs, connections that make it natural for them to work together.
I started my current campaign like this, going so far as to get teh players to describe the PCs' first "adventure" together before we actually began playing the game. Thus, by the time we started, they all had a sense of having worked together before.
Hopefully, this will generate peer pressure on this guy to have a non-homocidal PC. It might not do anything about the country music lovin'.
* Ask players to fill out a questionnaire about their PCs that goes into gory detail. Ask questions in it like,
"Did you have a nickname as a child? How did you feel about it? Does anyone still call you by that nickname?" and,
"What did you do for your last birthday? What do you figure you'll do for your next birthday?" and,
"Are you romantically involved currently? What do your romantic/sexual relationships, if any, look like? If you have never been in a relationship, why not?" and,
"Who do you most look up to? Do you know them personally? Do you have a mentor? Do you keep in touch with anyone who acts as a role model?"
Your goal here is twofold: first, by developing a social context for the PC, you'll discourage antisocial PCs. Second, you want to ask enough varied questions that a single schtick can't answer them all. Sure, he might have been called Tex as a kid, he might have gone to see Robert Earl Keen for his last birthday, he might have slept with all of the Dixie Chicks at one point or another, he might really look up to Willie Nelson -- but if he starts doing that, you'll have something to point at, something to say, "No!" to. You can sit down with him and say that you'd like to see other influences in his PC's life, such that it's not such a one-trick pony.
* Finally, talk to the players before the game and make it clear that you want it to be somewhat serious: while in-character humor is fine, PCs who are insane are not, and it's insane to risk one's life to satisfy a frivolous desire (finding shoes, listening to country music, etc.)
Hope this helps, and good luck!
Daniel
PS If you're interested in the questionnaire, I've got a couple of them somewhere in my computer that I can post; I think they're stolen from other Web sites, though, so I'd have to give credit to the Unknown Designer.