Welcome to the forum!
Here's the thing...he already knows it isn't good. He also knows you don't like it. He is smoking it anyway and most likely will continue to do so.
People rarely if ever stop doing something they like just because someone else points out that it's bad for them, or that the Bible teaches against it, or even that it's illegal. (Don't know if it's illegal in your area or not.)
This isn't to say that no one ever changes...but change will always come about from being convicted on the inside, not from outward pressures.
A lot of women think that if a guy loves her, he'll give things up for her, or do things for her, and he might in the short term. But, unless the change is coming from his own heart, he'll most likely go back to it.
This is why, when you are with someone, you take stock of all there is about them with the understanding that you get what you get, good, bad and ugly. You need to decide if you can live with all of it. And, you need to determine what things are "deal breakers".
Again, he already knows it's not good for him, but he isn't going to stop doing it. You need to decide if his pot smoking is a "deal breaker". If it is, break it off. If it isn't, sit down and have a good open talk with your parents and pastor/youth minister as to why you're OK with a guy who smokes pot.
One more thing...don't "threaten"...don't go the route of "If you don't stop, I'm breaking up with you." Threats are no foundation for a relationship. Accept him just the way he is, or move on.
