Gotta put my 2 cents in. Due to movies, tv and internet--kids these days are learning a whole lot more about sex at a younger age than us older folks. Please have a very open and frank talk with your son about sex. He needs to know the seriousness of possibly catching a disease or getting a girl pregnant. And I have read instances where a girl became pregnant without 'going all the way' but just heavy fooling around.
My son is almost 16 and very popular with the ladies at school. I'm not sure if he is a virgin or not--but my aim is not to judge his morality--my aim is to make sure that if and when he does have sex, hopefully he will follow my advice about using double protection, and that he keeps in mind that having sex with girls under 18 could land him a jail sentence if the girl's parents chose to pursue that course. Kids are going to do what they want to do when they are away from us, so I'm am a strong advocate for sexual education for young people.
As far as the 'Bi' issue, just make sure he knows that you will love him and accept him no matter what--kids need unconditional love and support from their caregivers.