Help With My Bestfriend. What should I do?

I hope to whoever reads this doesn't think I'm a complete bitch,but here it goes

For the past few weeks I've been feeling really annoyed with my best friend.
Mind you I love him to death, I've known him for five years and he's the best person ever.
Though like I said for the past few weeks I've been annoyed with him.
I can't have a normal conversation with him, like ever because he always refers everything to Lady Gaga.
He's overly obsessed with her, like his whole being is her and he never seems to understand the reality that he's a senior and that Lady Gaga isn't going to help him with his future.
Everything to him is a complete joke unless it's about Gaga.
I'm annoyed and there are times where I want to smack the hell out of him for being a samrt ass about everything.
He thinks he's just so bright and that he's the perfect child.

Everyone in my school except for those who are really close to him thinks he's gay-he's not, he's metrosexual-and I defend him all the time, but sometimes I think he doesn't appreciate it.

One last thing.
Things are getting to the point where I feel like he doesn't trust me as much anymore because he has his "little monster" friends who he's never met in his life.
He puts so much trust in them and acts like they've been his bestfriend for years.

It makes me sad.
All of it.

I don't know what to do.
I don't want to end our friendship, but I also don't want to hear about Lady Gaga every five seconds of my life and I want to have normal conversations with him like I used to when we first became friends.

I need advise.

What should I do?