Ok, that makes a bit more sense. It seemed like it was a "hit and run" based on him not being sighted.
Unfortunately, as the owner of the vehicle, you can be sued and held liable just as much as the driver of the car, under some circumstances. These circumstances include if your son was driving at your request or if he was running an errand for you. If none of this is true, then you are technically not liable, but your son will be.
If the other driver has gone through their own insurance company for repairs to the car, it would be best to talk to their insurance company directly to work out a payment plan as the insurance company will want at least some of their money back that they had to pay to have the person's car repaired.
If, however, you want to fight the case and not be liable personally for any payments (that means your son would be however), then you would have to deny the allegations contained in the complaint (lawsuit) against you. Depending on the value or the amount of money the other party is requesting, you will eventually go to trial. If you testify and present evidence that shows your son was not driving at your request, was not your employee and was not running an errand for you, then you yourself will not be liable to the other driver.
Do you have any questions based on this?