You need to give your son some motivation to finish school and to learn some job skills, and, ultimately, earn his own way. You will not accomplish this by not requiring something of him. I would tell him there will be no more money unless he re-enrolls in school or takes GED classes and either graduates or gets his GED, and then he has to get a job, even if it's working at a fast food restaurant, as a janitor, or whatever he can get. Then he might be motivated to get more education so he can get a better job.
If you want to continue giving him money, make him work for you and earn it first. This kid needs to face reality, the sooner the better.