Son wants to meet bio-Dad...he doesn't know bio-Dad is a rapist?

Well, what type of relationship do you have with the father? Obviously not romantic or sexual, but are you on good terms, decent terms, or hateful terms? If you're on good terms, then I don't see the harm on him meeting his biological father, although the explanation should wait a bit. However, if you can't stand him, then it probably won't be a healthy situation for your son to be in. Being caught between two spiteful adults that he knows are his parents can cause psychological and confidence issues as he gets older, especially since around puberty is when kids are at their most confused and least confident stage. If you're completely at a loss, I would suggest seeking out a psychologist. Besides giving advice, he/she could also help facilitate the introduction if that's the route you decide to go. Either way, good luck!