One more thing I'll note. I sent an email to a support group I found earlier (parents of gay/lesbian kids), basically repeating the OP here. This is the reply (in pertinent part):
I'll try to address your questions. First, don't confuse sexual orientation with sexual acts. Sexual orientation is who you are attracted to, who you fall in love with, not necessarily who you have sex with. If you think back to your own childhood, I'm sure you can remember having a crush on someone at a far earlier age than you knew about sex, that's sexual orientation. You say your son has had no romantic experiences, but he may be attracted to a schoolmate without ever saying anything or have a crush on a pop singer or movie star, enough to make him understand his own desires.
As for bisexuality, I don't know if you have heard of the Kinsey scale but it's a way of rating one's attraction and there are indeed people who are 100% heterosexual or 100% homosexual but then there are others who fall somewhere in between. Given that we live in an overwhelmingly heterosexual society, most of them end up in a relationship with someone from the opposite sex because it's what's expected, but not always. And you are correct that teens may sometimes identify themselves as bisexual because they just aren't ready yet to come out as gay. Sometimes you will see the phrase "gay, lesbian, bisexual and questioning" with the last category covering some of this gray area as well.
Finally, you ask about who you should speak to. If you wish you can bring it up with your son as I did with mine but he may deny it or make another excuse because he may simply not be ready to come out to you. I've heard that coming out to parents is the hardest thing that gays and lesbians may ever have to do; even if there isn't the fear of rejection, there's the knowledge that they will be causing their parent some pain. You may wish to bring up the subject in a roundabout way, perhaps discuss gay issues with him and then mention that if he or any child of yours were gay then it would be fine with you. I can tell you that once the secret is out your son may be relieved; after all, he's been living with his secret for a while now.
I do agree that you should not tell your wife. If your son wishes to you can support him in his decision but it really should be his choice who knows. This, unfortunately, puts you in the closet. Now as your you, speaking from my own experience, it's easy to say to our children that it doesn't matter if they are gay, but at some level it does matter. If nothing else we need to adjust all those dreams we had for our children and I think, too, there is a part of us as parents who think we know all about our children and now we find there is an important part of who they are that we didn't know at all. I found that reading helped me adjust and also attending PFLAG meetings. There are many good resources at
www.pflag.org including recommended reading lists.