Oh dear, it's hard being a parent of a teen of this age group who seems intent on mucking things up, but unfortunately I think he is going to have to learn for himself and possibly the hard way. You can't make him change his attitude; the only thing you can do is what you're doing really i.e. lay down the expectation that he either attends college or gets a job while he lives under your roof. And don't make his life easy in the meantime by providing spending money etc.
My son is 17 and has just been expelled from college, for bullying, and his not showing any remorse for his actions, he was on his final warning after he was moved to a specialized programme, (well that's what they made me believe) after he got kicked off his first course for calling his tutor an offensive name...
So I told him that he has till the first week in January to find himself a job (knowing that he will not find one) or appeal to get himself back into college.
I am really concerned about him, and his lack of interest in making positive decisions about his future.