Don't worry about his musical tastes and interests, it is his life afterall, he should be free to like whatever he likes. Just cause YOU don't like it doesn't mean he can't. Would you like it if the situation was reversed and you were the younger one and he was like "My punk ass brother listens to gay techno and rock and roll. This has to stop!"
But DO worry about him wanting to buy a gun... that's just stupid. Don't let that happen - you don't want to hear about a police officer that pulled him over because of a busted tail light that he shot and killed cause he panicked or any of the absolutely ridiculous stories you here in the news today related to kids with guns.. or some kid at school makes fun of his haircut so he pulls out the gun to scare the kid and it goes off and kills the innocent girl walking by on her way to class. Don't let that happen.