My son lives at school and comes home on the weekend and has his own liability insurance on his own car and he drives mine and has an accident which ins co pays?

Your auto insurance policy would cover this situation. Auto policies cover the automobile, not the driver. While it is true that there are "listed drivers" on auto policies, anyone that you allow to drive your car (and is legally able to), is in fact covered. It also goes back to the insurance concept of "insurable interest", meaning you must stand to lose something or suffer some type of loss to trigger coverage. As an example, you could not take out a life insurance policy on me, because if I were to die, you would not be impacted, financially or otherwise.