What about showing him all the different breeders, add them as favorites under a folder labeled Ball python, and show him all the morphs, how many snakes, and show him the caresheet, Saying all these breeders, with these crazy looking snakes worth thousands of dollars, keep them all the same way.

A pretty animal always is a good selling point,
NERDS site may be a good place to start.

Being near his age, im supprised he listens to everything his dad tells him, Personally, When I got my animals, my care is what i beleive and research, but im also independant on that stuff, and really am stuborn and a perfectionist.

If the caresheets doesnt work, shock value may help, find a picture of a ball python very weak and sickly, and say that his will look like that without following these care concepts. Not ideal, but id at least look into things if someone said that to me.

He sounds like him and his dad are just being Plain Ignorant, and stuborn and know-it-alls, and im sorry you are stuck trying to figure the situation out, I have a few neighbors like that, but not as bad, Id just like them to read a few caresheets and realize that maybe there adult dragon needs a tank larger then 20g, or something of the sort.

Just dont fret too much over it, you cant change the world, But its worth trying in some cases, If things dont work, at the very worst, the snake wont make it, and he may not get another, Sad but its life.

Hope things work out,
Ben