Neat story ST.  I wonder if the right thing to do is to come 100% clean & explain the honest mistake.  It's one thing to surrender the gun at the border, something else to smuggle it in.

I do wonder if they would let him pick it up on the way home.  Maybe Can customs would surrender it to the US officials.  There should be some provision for him to surrender & recover it.  After all, it's his legal property and any issues in the US are between him and the US.

There's no licence issued in any US state the supercedes the laws of the nation you are entering.  I'm not a hunter, but I suspect there's paperwork involved in brining guns from one country to the other.