Kishka,
Part of what you are seeing is the way an abused boy's sense of sexual boundaries can be totally wrecked by what is being done to him. His whole sense of who he is sexually is scrambled up and confused.
These feelings can easily continue in one form or another into adulthood, and it's not unusual for a confused survivor to wonder if he's in fact gay. When I first started to face what had happened to me, that's one of the first things that came to my mind.
If you discovered things and asked him about them, then he probably felt completely shocked and insecure. When someone feels cornered like that closing down emotionally is a pretty common defense mechanism.
That of course wasn't your fault, and as you say, this isn't just about him. You are hurting too!
It's complicated dealing with a situation like this, but I would suggest that you try to reassure him while at the same time asserting your own concerns as well. He needs to know you love and support him, but at the same time he needs to know that you have feelings and needs too.
It's a tightrope really. He has to feel he isn't being pressured or cornered, but at the same time he has to know that your concerns aren't less important than his. I can tell you that I didn't start talking about my abuse issues with my wife until I was under tremendous pressure - not that she meant it that way, it's just that I became sexually dysfunctional and she took that as a sign that perhaps there was "another woman". I was also becoming increasingly irritable and detached, and finally she broke down and told me that whatever it was that was bothering me, it wasn't her fault. It was then that I started talking to her about the abuse.
It's best if real communication doesn't wait that long, of course. I just mention this to illustrate the difficulties.
Looking back, what would have worked better for me? I think I would have opened up sooner had I feld more certain that I wasn't going to be judged and abandoned. I genuinely wondered if anyone could ever believe the shit that had happened to me. If my wife knew, wouldn't she just be disgusted and take off?
I guess what I'm saying is I wish my erratic behavior had been met with more positive encouragement and support, but I do know that's just how it looks from my perspective. Viewed from hers, and her fears that there was another woman, it of course all looks entirely different.
Just some thoughts.
Much love,
Larry