Bruises mean burst capillaries which are the smallest blood vessels and that happens when we bump into something as we all know, but cracked ribs and damage to internal organs is a fear when someone is hurt in the midsection. If it were my child I would lay him on the couch and gently move my hand around his stomach and chest area pushing lightly and ask him to tell me exactly where it hurts. That should give you an idea if you should worry or not. That's what the doctor would do, but of course they have the expertise to know if they feel something wrong.

As far as the violent boy I wouldn't take my eyes off him or let him into another room without me as you said, watch him like a hawk. Too bad there were no witnesses but catch him next time and I would definitely let the parents know about how this incident happened. Document it in your logbook or whatever system you use and keep records of incidents. If you need to send a warning letter to the parents that they need to work with their son on his anger management you will have everything in writing to cite to them.

I hope your son is ok!