As a starting point, what is it that you or he wants? The only time these credits are worthless is for people who can't use them and it sounds as though he can.
Why even ask here until you know the facts? What did the document he signed say? If he agreed in writing to whatever it is that DL offered, perhaps even specifically to flight credit, that is the end of it. If your son wants to sue the individual DL employee who made the commitment, he can do that.
But, if the document says that the son gets a choice, then that's what DL owes him.
Presuming that the son didn't snap a picture of the document, I would simply ask the next GA you bump into at a non-busy time to print one off for you so you can see what it says. It's always possible that the son got a different form, but it's a starting point.
Once you have the form, you will know whether you pursue DL in writing, file a DOT complaint or he sues in SCC.