Is he talking proof of registration with the gun manufacturer or what? There is no firearm registration law. Where is this guy from?
Any way it sounds like you have proof that you tried to resolve this issue and any futrher issue with liability for the firearm is on him since it's in his possession and he refuses to co-operate.
He's from Indiana. I tried explaining that point to him but he refuses to listen to me. I suppose he naturally has to be difficult given the situation, nonetheless I hope the free firearm turns out to be a blessing for their home, I'm a kimber kind of guy anyway .