And to close, even if you agreed to "go along to get along" (either because you decide to, or because in some alternate universe he's actually sued you, won, and you now need to dispose of them to his benefit), the right thing to do would be to cheerfully tell your son that you are going to take them to your local FFL, and wish him "good luck" in getting them re-transferred back. The worst thing that can happen to you at that point is that you're freed of their burden, and the best that your son can hope for is that they sell quickly at the FFL so he can receive the benefit of the sale, less comission to the FFL. Last time I checked... well, he's gonna get scrrewed by the FFL on that part of the deal.
In any event my free and "worth what you paid for it" advice is:
"Don't sweat it" to you,
"If you want to sue me for them, go ahead" as your next reply to him.
Just never *ever* transfer them to anyone (including ex-wife) "not via an FFL". It's cheap insurance, especially bearing in mind that the buyer pays the fees. It's a no-loss-possible game for you.
If it were me... <insert best Bart Simpson look here> "Guns? What guns?"