How can I ask him about drugs?

I work in a preschool. A teenager comes there four days a week to practice. One day a week he goes to his preparing school for misbehaving students. In august he's supposed to restart at the real school again. I'm wondering if this guy is using drugs. He smokes cigarettes and that often lead you straight to drugs.. He has difficults with coming in time in the mornings and he has been at home for too many days already. I want to ask him if he uses hashish or something but I don't think he wants to tell me the truth. How can I ask him to get the sincere answer?☺☻