There should be no shortfall, your contract of indemnity insures your son up to the market value. It is your son's car and your son's claim. Do not be bullied into anything. Firstly you need to establish if the car is indeed a total loss. A vehicle is only ever a total loss when the repair cost exceeds the market value. Request a 'Full Engineer's Audatex Report'. Do not let the insurer try and fob you off with the 'summary' report. Is the cost of the repair less than then market value? If it is, insist they repair the car up to the market value. Do not take no for an answer. If the repair costs are more than the market value, then the car is indeed a total loss. Control your asset, do not let the Insurer take control of the car until you are happy with which way the claim is going.