i would start with the bank, he closed the account and the bank is handling things properly by returning the checks. they may be able to help with the stores or give a suggestion of who could. also the major chain stores all have somewhere you can go over their head, if the store manager won't help, find the corporate offices. your son can't be held liable for it and can probably do something about harassment if they don't back off him. just a thought.

lost

edited to add, the stores are responsible for checking the ID of anyone who uses a check, therefore it is their own problem if they screwed up and let something get past them.