First, I would like to say how badly I feel for your son. These scumbags are the exact reason, I don't trust anyone.
I also feel, that you letting their permenant record screw them is the best payback, but if they are minors, it will probably be cleared once the turn 18.
However, this is how I would handle the situation. I would make sure that all the kids in their school knew the did this to a handicapped kid. The rest of the people in thier school will make their life a living hell. For example, when I was in HS, there were 3 guys who would always pick on a kid in the special ed classes (he rode my bus) well, me and my buddy got sick of seeing this everyday, and eventually beat the crap out of them on our way home from school. You see, we had the perfect aliby "we were walking home and intervened when these guys started messing with him"
Turned out they never told anyone because they new that it would have come back to them again. For every scumbag out there, there is an exact opposite.
I hope eveything works out fine for your son, and family.
And keep us posted as to wat happens!