To be honest he's probably thinking why should he. He can get sex outside your marriage and you forgive him, your paying all the bills so all he has to do is worry about his school. He's having his cake and eating it too while you support him.
He is USING you. I will bet as soon as he finishes school & gets a job in his chosen field he will be out of there.
I am not trying to be hurtful, this is just what I see from the short post that you have written.
Good luck making the best decision for YOU