between what your sons own insurance will cover (what we call a “first party” claim) and what the at-fault drivers insurance will pay for (what we call a “third party” claim).

If your son is talking to his own insurance company, and if he has comprehensive coverage, then his own insurance is required to pay for the value of the car. The value of the car is just what it was worth on the used market.

If your son is talking to the at-fault drivers insurance company, then they’re required to pay the same, but also potentially more - loss of use, lost income, etc. These are potentially things that the at-fault driver is required to pay, and the at-fault drivers insurance would be required to cover.

It isn’t actually very complicated. Call your own insurance first. Negotiate with them. Get some evidence of what the car was worth on the used car market. The insurer will negotiate with you. Their opening offer is not their final offer.