If your 18 year old unlicensed son drives an insured car with the owner's permission and an accident occurs does their insurance cover it and what is your liability since he is 18 you live in CA?

The insurance policy on the vehicle should still apply. Since the person responsible for the accident is 18 the liability should end with the 18 year old but I am not a lawyer and don't live in CA so I can't say for sure that I am accurate.