If your son is buying the used car at a dealership then he may want to negotiate the windshield being repaired or replaced as part of the purchase agreement.

If this is a private sale for a car in as-is condition then that would mean your son is buying the car with the broken windshield for him to repair unless the person he is buying the car from has agreed to make an insurance claim or otherwise repair the windshield before he takes possession of the vehicle.

Your son would not be able to make a claim against his comprehensive insurance coverage since the broken windshield is pre-existing damage before his insurance was placed on this newly purchased used car.