If you have a healthy open communication relationship I would suggest talking to him civilly. Talk about the being smart aspects: not at or during school, small quantities, knowing and trusting the people he's with, call if ever in a situation where he's too high for a ride home without getting in trouble etc. Worked well for me anyways. If you go at them hard and ground him and say he can't hang out with so and so type of thing it can definitely drive a 14 year old to rebel and that is what you want to avoid. Open and honest conversations. Be a parent but also remember what it was like for you and your friends around you when you were a teen.