What should I do about my (bisexual/gay) best friend?

[A]

I'm a straight male and in highschool. I've been in a tough situation for awhile. I have this really, really close friend. He's like family to me. Recently, I've gotten into a relationship with a girl, and about the same time my friend became suicidal. All he wants is to have me and him as a couple, and that he gets really really hurt whenever he sees my girlfriend with me or he hears about her. He's been threatening me with suicide and they're not just threats. I've had to actually stop him from doing it on many occasions. I've gotten his parents involved, but he's a very good liar and might be able to convince a doctor he doesn't need help or anything. This has been going on for a couple months, and it's getting to the point that he is giving me the option of (1) breaking up with my girlfriend and trying to be gay (which he's convinced is possible because I've never tried to be gay so "I might like it") or (2) letting him kill himself.

He's a very close friend and I don't want him to kill himself, and I would like to find a way to remain friends. At the same time, I'm not big on the idea of him screwing up my love life. Any advice would be nice.
Oh, I did not know he was gay/bi or anything until after I got into the relationship.
Also, his family does not know the detail that he's gay/bi, and it would probably be a problem if they did.