Did the police submit the case to the DA for possible charges?
You may have a claim; because every situation is unique and based on the facts surrounding that situation, I'd recommend you contact an attorney in your area who specializes in education or administrative law. Many lawyers provide initial consultations at no cost, so you'd have a good opportunity to review the facts together and discuss possible next steps / the viability of a lawsuit.
Thumbs-up to your son for walking away in an attempt to manage the situation. I'm so sorry he was injured in the end and I hope he is recovering/has recovered well. Good luck to you.