I am so sorry your son has gone through this. He is deeply fortunate, all the same, to have parents to pay attention to this and to help him recover. Wish we all could say that--! Blessings to all of you on your journey.
My comment may seem off-the-wall, but I make it seriously, for you to use as you please.
Have you considered filing a civil suit against the parents? I am not a lawyer, and I do appreciate the pitfalls of entering the system from yet another angle. However, sometimes people take the possibility of losing money much more seriously than they take criminal charges, sadly enough, and since the parents cannot (I assume) be held criminally liable in the form of negligence, I wonder whether you don't have another course of action open to you this way. It's long past time for them to be paying serious heed to the danger their son poses to other children, and highly irresponsible at best for them to continue to pretend otherwise, and grant him this kind of access. Their tacit endorsement of this website seems indicative of some serious dereliction.
I'm just a poor wayfaring stranger, a million miles away from home.