The first thing I hope you did was burn his ear off, it is a criminal offence to not exchange details if you have a collision. That's the negative. The positive is, as he was on private property, the Road Traffic Act doesn't apply and there is no duty to exchange details. Even though it was wrong that he didn't, it's unlikely that the police will take any action. It would take the owner of the other vehicle to press charges for anything to happen. I would suggest the best way forward is to ask the police for the other parties details, so that you can contact them to exchange insurance details. I would then ask if they would mind you looking at the "alleged" damage your son has caused, as you may wish to pay out of your own pocket on a "without prejudice" basis. You can then see if your son has damaged the vehicle or not and work out the best way forward.