When someone accepts a check - they are supposed to verify the identity of the person who writes the check - it is their responsibility in order to protect themselves.
The store should not have accepted the check.
The store did not carefully verify the identity of the person writing the check.
What does your son's bank say?

The person who stole his checks and who is writing on them (forging his signature) is breaking the law and this is the person who is responsible. Not your son. Forging a signature is against the law.

It's like when I accepted a check from a person who then told me - "oh there's no money in the account" - I am stuck with the check that is not cashable, unless I can convince that person to pay me cash, or unless I go to the police/court in order to force the person to pay.
I accepted the check. It is my problem now.