First of all, CA is a comparative negligence state

This means if the jury finds your son to be 10% liable, he will have to be responsible for 10% of the damage. Secondly, your son is a minor and he is subject to a different standard as an adult, which may work to your benefit in assessing where he was negligent at the time. I would say the best solution is to settle the case with some payment to the other kid. After all, they will be in the same school for a while. But I do agree that it is unfair to ask you to be responsible for all the damage.