Am I supposed to be liable for my second driver’s accident claim?

Unfortunately that's the risk you're taking when naming additional drivers on your car insurance policy. Your no claims bonus will be reduced if they have an accident where they are at fault. The same thing happened to me and yes it is not fair but that's how the insurance industry works.

You will definitely see a huge increase in premiums if your NCB has been reset to zero but remember that the accident remains on your son's name and not yours. As far as you are concerned, you still have a perfect driving record, that is, no accidents/no claims on your behalf, the only difference now is that you don't have any NCB to bring your quotes down.