ooh wow, I can understand why your son was so confused...and that is not ok.

I would think it must be time for a sit down withn you ex husband. Tell his behaviour is unacceptable adn ask him why he has changed his attitude when you were both onthe same page for so long.

If you haven't done it already I would explain to your son that guns are still not ok, despite it being given him by his dad, are not allowed in your house and thow it in the bin. Explain to him that he has the right to say no to his father next time he asks him to do something he knows is wrong, that if he isn't comfortable with something he has the right to say no to his dad.

Reinforce why guns are bad and that he has a choice to do something wrong or stick to his and his family's values...make it about him and his decisions rather than bad mouthing his dad.

Good luck it sounds like a very tricky situation