I know how much it suucks but I think you and your son need to prepare for S to go to a different school. One thing I have learned is there is no convincing a MLCer to do the right thing.
I wish I could say something to make it better but you are better off spending you your time trying to figure out how you can pay for it then convince him to pay for it.
Your H is an a$$ - he is not gonna come around to seeing how wonderful your S education has been for him.
---- M 39 H 35 D5,D4 M 4 T 9 ILYBNILWY 5/18/11 Left 7/11/11 Divorced 12/1/13