A little while ago at one of our kilt nights we brought several different fur sporrans, Gary (Wompet) was there and was easily able to identify my Sealskin sporran, Gary works at the US Canada border on the US side, he was saying they would confiscate the Sealskin Sporran if I were to cross over with it. I therefore also have a Bovine one for going to the states with. As for once in the country it would most likely be fine, however as I've already said getting it through customs will be a problem, unless you can prove that the sporran is a certain vintage, and I'm not sure how old it has to be to be able to get a pass, I think it must be at least older than the Marine Mammals Protection Act, that was enacted on October 21, 1972.
As to how to get it back from your son, I'm not too sure how you are going to do that.