Should we retain a lawyer to fight for damages through other parties insurance?

Yes. Contact a local and reputable personal injury attorney as soon as possible. Although there is no indication that your son suffered physical injury, if his vehicle was totaled the at fault party should be responsible for those damages. Additionally, an experienced attorney will be better suited than a lay person in dealing with insurance companies and will ensure your son gets the most money possible. Most personal injury attorneys operated on a contingency basis so you will pay no money up front. Good Luck.

A lot of Lyft and Uber drivers are learning this hard lesson. Yes.

Your son is in a tough spot, that is a lot of money. He should speak with a local personal injury attorney about the issue with his gap insurance. Bring a copy of the policy and any letters or denial from the Gap insurance company. Don't settle with the other party's carrier until you speak with an attorney.

can we presume the at fault parties insurance company made an offer to cover this vehicle property damage but it is below the loan amount due on the damaged vehicle? The denial of Gap coverage is likely justified if the accident occurred the covered use of the as described in the policy. An attorney can help ensure the property damage settlement from the at fault carrier is maximized and recovered. I wish you the best of luck in this matter.