This might be disturbing to you, but you sound like you're not going to react rashly. You are well aware he will always be your son, no matter whether he ends up gay, straight or bi. He needs this reinforced, as he will know some parents let their child's sexuality dominate the relationship they have with that child. Even if it's obvious, actually telling him that at the end of the day you don't care if he likes boys or girls or both, as long as he is safe and happy.
The "safe" bit gives you an angle to talk about sex... you can ask him right afterwards, or refer back to the conversation later... "My desire that you'll be safe means I want to ask a few questions, is that okay?" The "okay" is a bit of a cheat - whether he's okay or not with being asked the questions it is FAIR that you can ask them, but keep on validating him in that way. This can open up discussion of legal issues and sexual health issues, which however uncomfortable it will be needs discussing.
You will feel better knowing; not knowing will simply mean your fears will have no borders or clear shapes.
Personally speaking, the only actual homoerotic experienece I had was at school, and consisted of caressing the thigh of another kid with the end of a pencil whilst he did the same to me, under the desks in a handful of lessons. Massively arrousing it was, but I clearly remember not even thinking about it in term of liking boys; it was all about the physical sensation. But on other levels I was weirded out by it enough to block the incident out for years. Now it doesn't bother me in the slightest, as should be the case. In no way do I wish to imply I feel my heterosexuality as an adult is a better option than other sexualities, just the right option for me.
Whether it as something as normal as finding out your body reacts a certain way, and your son is trying to find a context for this and coming up with the label 'bi-sexual', but it's not a true indication of sexual identity, or whether he is breaking you in gently for the "I'm gay" conversation, or whatever, you deserve a lot of credit for having a relationship where your son feels free to open up to you like this, as has been commented on.
He trusts you with something massively intimate that you could turn in his face and make his life a misery with. Carry on behaving in a fashion to maitain this trust as you obviously have done in the past, and whatever his outcome in terms of sexuality, you''ll have a good realtionship with your son.
Of the gay people I know well enough to comment on, the impairment of the relationship with their parents is one of the most common problems and often the greatest source of grief, and you have a very good chance of avoiding it due to your attitude and open mind.