hello,
I just want to make it clear that you don't have to be sexually active to know that you are attracted to someone. You, as an assumedly straight woman, can look at a man and say, "Hey, he's hot." You probably have known that guys are attractive all your life. Much the same way as, before I was married and started having sex, I knew that I found girls to be attractive, despite having never slept with anyone, let alone someone of the same sex! Much the same way, bisexual people can look at a girl, and say "Wow she turns me on" and then look at a guy and say, "Wow he turns me on too" Also, attraction is not just physical. There is emotional attraction based on people's personalities involved. You didn't fall in love with your husband just because he was good looking and good in bed, did you? So why should sexual activity be the be all and end all of your son determining his sexual orientation and identity?
Thanks for your post. I would not worry too much about what to say, focus instead on listening to what your son has to say. Chances are, he is not looking for "an answer" per se, but just to be heard. As far as what he means by "bisexual," it is not clear, and it may not be clear to him either at the moment. That may be something he will explore (internally) over time, and if you are as open to listening to him as it seems you are from your post, he'll keep you in the loop.