Given what you say about what happened to him when he was young and how he's acting out, and admitting a sexual attraction to his Mom, I'd say it's time for professional counseling with a psychologist who specializes in pediatric psychology before this goes any further. That's just my take on it, people do have wildly different sex drives, but if he's doing this in public and you know he was forced when too young to understand to engage in sex, and I'm going to say here, it's not clear that someone that young can actually perform, I assume some can and some can't, but he's not only trouble, he's also obsessed at a pretty early age. It can't hurt to have him see someone who knows more about this. Peace.
Absolutely, professional help needed. He was molested even if by another child, still molested. He doesn't seem to recognize normal boundaries. Mom, at school, in public? None are okay. So, work with a professional asap.