I responded earlier Crois, and erased as I'm not feeling great right now, and I wasn't sure I was making sense.
I am not a parent, but I am a recovering addict. Grass. For many years.
In my experience, it is no different than alcohol. Being confronted can cause denial, and anger.
I think it is a good idea to talk to your son in a comfortable environment for both of you, and simply ask him if this is a problem for him. Open the dialogue....give him a chance to hear you and know that you care, and to think about this. Sometimes just knowing that others are worried can be a catalyst for change.
Just my thoughts. ♥