I need advice.

It is a difficult thing to talk about and he probably just doesnt know how to say it, if in fact he is bi or gay. I imagine, for most, telling a parent would be the hardest. Most people I know can't talk to their parents about sexual things very comfortably. My experience was very different but I have talked to others and it seems pretty normal, especially if he has not yet moved out on his own and established his own life, that he would be cautious about what he tells you.

You could find an interesting gay novel and read it when he's around so that he sees you reading it and then tell him about what you're reading and express positive words about the gay experience. If your son hears you saying that gay love is a beautiful thing, he might feel freer to talk