Listen, your kid got exactly what he deserved. Children need to learn that when someone can kick their ###, they need to shut the ### up when told to. If your son can't understand that much, I would recommend military training, instead of some prissy "Tae Kwon Do". I mean, ###, at least put the kid in kenpo or something manly.
From everything you've said, I believe that law enforcement is too uptight about this sort of thing. The kid had it coming. Nothing you can say, no excuse you can give, and no "opinions" by a police department, judge, DA will ever change that.
The man may have been unstable, may have had aggression issues, but if I'd been in the same position I would have smacked your son upside the head. Now teach your son some goddam respect, since your silly little "martial arts" obviously isn't enough.
