You will need to carefully inspect the paperwork sent granting permission to short sell. In most cases, this is a statement that accompanies the payoff quotation that would have been required by the attorney or title company completing the sale. Some lenders clearly indicate that the unpaid debt isn't going to "go away", but is ratehr going to become an unsecured debt like a credit card . . . this is particularly common if the property sold was NOT a primary home, but an investment. In other cases, the paperwork clearly indicates that the debt will be considered satisfied in full and no further $$ is owed.
Start there . . . if there was a short sale negotiator involved, he or she will have this paperwork (and should have given it to your son!!!) . . . if not, the attorney/title agent and/ or real estate agent will have it. There had to be a statement issued in order to sell and discharge the mortgage . . . so track it down.
Let us know what you find . . . and then we may be able to be more helpful.