It depends on what you think about weed. It can be beneficial as a treatment for some conditions but I believe that it's not harmless as a recreational drug, especially the younger you are.
If you don't think it's beneficial to him you might gather some data to present regarding the impact it has on youth.
Also, it depends on your relationship. Does he trust you, does he obey you, what's important to him? I think you'd want to craft a strategy based on answers to these questions. Would he respond to a "trust me" conversation, does disciplining him help him, or is he reachable through an appeal to data or an appeal to how it could impact his goals?
If you don't think it's beneficial to him you might gather some data to present regarding the impact it has on youth.
Also, it depends on your relationship. Does he trust you, does he obey you, what's important to him? I think you'd want to craft a strategy based on answers to these questions. Would he respond to a "trust me" conversation, does disciplining him help him, or is he reachable through an appeal to data or an appeal to how it could impact his goals?