Your born gay. It isn't a mental choice. That'd be ignorant to think so. He just is young and doesn't have anyone to be gay with. He is worried everyone he knows will shun and judge him...hence why he says he is bi so he will fit in. Just accept him dude. He has been gay the entire time you've known him since his first day out of the vag. You just didn't know. Now that you know...it doesn't make him different unless you let it.
My best friend is gay....he told me and explained it as i have to you.
Be a man and love your family no matter their choices. Sure he might fuck dudes...but he isn't trying or wanting to fuck YOU so why should it matter? It isn't like he makes you watch him and his gay friends. Lol.
I like you dude. Don't turn into an ignorant fuck because he's gay. Plus chicks LOVE gay dudes....specially hot ones. You could creep on his straight chick friends...he can put in a good word...tell em you got huge junk and shit.
If you help him out now he'll help you out when he is surrounded by poon he doesn't want.