I would suggest agreeing on a compromise with him. Let him smoke on the weekends, stay clear on school days. The important thing isn't whether or not he uses it, but that he uses it responsibly. Some people just aren't responsible enough to know not to smoke during school, before tests, before work etc. I'd really emphasize that you'll be cool with it if he proves to you that he can still keep his life together while smoking. Just really lay it down on him that he's determining whether or not he's earned your trust and respect.