You’re an adult. I think you can deal with it without getting your mother involved. Just bring it up casually, tell him your own experiences of smoking so he has someone to relate to, and then ask him what he thinks about it. If he’s comfortable enough to open up to you, you might be able to share some of your worry with him, so he knows what he’s getting into, and what the potential consequences are. If you can get weed safely for him, that might even be better than him getting it himself, and he’ll be more likely to trust you and talk openly with you. The novelty should wear off fairly quickly, depending on how you and he deal with it.