He is well old enough to understand that you are concerned about the money that will be stopped, but equally it is not his job to support you. He does however need to be able to support himself.
Tell him that you stand to lose income and that has to be replaced. If he can get a job then he is welcome to stay at home and contribute. Otherwise, if he is staying at home then he better buckle down at college or leave and support himself in whichever way he sees fit.
He is effectively an adult now so lay it straight on the line.