It would be helpful to have more facts, but in the pure sense if your son was negligent and caused damage he is liable. By way of example, if he fell and damaged a parked car he would be liable for that. The fact that the car was moving is irrelevant to the issue of liability.
I concur with Mr. Garner. Your son may be liable if he was negligent and caused damage to the vehicle. Unless something unusual happened, the property damage to the vehicle was probably minimal. The insurance carrier may be willing to negotiate a favorable settlement since it may not want to file a civil action against you for minimal property damage where your son sufferred injuries which kep in the hospital for weeks. If you own a home and have umbrella coverage as part of your homeowner's insurance policy you may want to tender the claim to your insurance carrier. You should consult an attorney to discuss the particulars of your matter and for any specific legal advice.