One thing you can try is threatening him with filing a lawsuit, or better yet if you have a friend who is an attorney and will do a little pro bono work for you just have the attorney send the dealer a little note telling him about the little lawsuit you will file if you don’t get your FAL.
Maybe the simplest thing would be to have a little chat with his father since you stated you have done business with both him and his son in the past and this is the first time you have had this kind of trouble. And if you don’t get a satisfactory answer, tell him you would like to solve this problem with going to court or you taking any other action. If that doesn’t work then maybe get a lawyer and try to work it out without going to court and make sure he pays for the lawyer. You can also complain to the BATF, I can’t believe I just typed that, but it might work. Dealers do not like any BATF entanglements, and that is for sure.
But just don’t let them get away with pulling this sh*t on you.