OK, guys, let's all get back to the basics here. This one has strayed all ove the road.
Nana - please tell us how old your son was at the time this happened. Also what state did this happen in.
Also, Nana, I know you want to be supportive of your son, and that's fine. However, your son did this once, it is not out of the realm of possibility that he could do it again. As stated before, it's probably best that you don't ask anymore detail than you already have. You also need to come to grips with the fact that he could have done this more, but does not want to tell you. I'm not saying that he has, but you need to remember that what he did was wrong and illegal. It doesn't matter if he was a good boy, etc, this has happened. You need to come to terms with the fact that he's in VERY serious trouble here, and as part of trying to deal with the problem, you need to also keep your mind open. Let me put it to you this way. If the 13 year old herself had told you this happened, without you hearing it from him, would you have believed her? Or would you have said he would never do anything like that?
Another question (just to clarify) did he start the therapy and suicidal thoughts right after it happened, or after he got confronted by the police?
As for the girls current sexual activity, other than what was said about being damaging to him, it wouldn't matter. If she's sleeping with guys her own age, then that is something that would be normal teenage activity. Teens do have sex.
The bottom line is he needs a damn good lawyer if he's going to get past this one, and given what you have told us, he probably doesn't have a good chance of beating this.