hummingbird, you can post as much as you like. Of course you're concerned about your son, but I'm afraid it's out of your control, unless something you're doing is actively helping him. I actually understand his argument about not working but taking classes instead. As long as he's still achieving good results, he will be gaining qualifications and using your support wisely.
Maybe you need to think about what you will do if his results begin to suffer. Will you go on supporting him?
Like any parent, you don't want to see him struggling with substance abuse, but this is a struggle he will have to work out for himself. All you can do is stop paying for it if he's no longer committed to his studies.