My first thought: sue, sue the pants off ‘em.
Second thought: move on gladly.
Third thought: maybe do both.
These days, employers can’t force you to leave your civil rights at the threshold, unless they’re your 2A rights and some of your 1A’s. So he probably has a case, but why would he want to stay with that job?
As others have said for other cases, it's better to be fired by one than carried by six.
Under New Hampshire law employers can fire you for any reason and without notice so he wouldn't have grounds for a suit. But he's alive and that's the important part. Finding another job as a convenience store clerk probably won't be that hard, but I suggest he read the policy manual very carefully before accepting it.
The State cannot tell you that you are able to carry.
A private employer is well within his rights to tell you not to bring guns to work. In fact, the owner’s insurance company probably voids any thefts or damages if the employee pulls out a gun and shoots someone.
I know it sounds bad, but it is there to protect the employees and customers. Having worked at a bank where two people were shot dead in the 80’s, I can tell you that it is better to just hand over the money and be done with it.
If this is your home, the story is different.
I have to say that I probably wouldn’t have fired this guy, but I certainly would have suspended him.