I am a gay man whose best friend for many years is straight. He just found out his son is gay and he isn't dealing with it very well. Frankly, I'm flabbergasted, as he knows I'm well-adjusted and happy. No, he hasn't talked about sending the young man -- he's twenty-four -- in for shock therapy or anything like that, but he keeps trying to convince him he's not gay and gets angry at me when I tell him that his son seems pretty certain. I think this is the end of a long-time friendship. Sam.
Sadly, there are many straight and straight-identified people who -- on an intellectual level -- are perfectly supportive of gays but on a deep down level have issues to deal with, especially when it relates to themselves or their loved ones. Since I assume you've been openly gay with him for years, it's unfortunate to say the least that he can't deal with his own son's sexual orientation, but this is not as uncommon as you may think. Part of it may be his concern for his son's welfare (discrimination) and health (AIDS is still seen by many as a "gay" disease even though it shouldn't be), but I sense that on a deeper level his reaction is blatantly homophobic, especially as he's not being at all supportive of his son.
He may come around when he gets used to the idea. Or he may not. It just goes to show that often gay people are merely tolerated by some straight friends instead of truly being accepted as equals. Don't give up on this long-lasting friendship right away -- tell him how you feel in no uncertain terms -- and be supportive of his son, but if things don't show signs of improving this is one "friend" you can do without. Hopefully his son's coming out may help him to understand you better.