Advice please.

Okay so my fiancé has had lots of problems with all kinds of drugs in the past. He's gotten in trouble with them before and almost went to prison for it, but they let him go to drug court instead. Well he just recently graduated from it. We've had the discussion back in August when we got back together, we dated 4 years ago, that i would leave him if he ever did any kind of drugs again because he's chosen weed over me before and he's a completely different when he's high. Well after he graduated drug court he had one of his old pot head buddies come over and I heard them discussing talking about smoking weed again. I looked at my fiancé and asked what they were talking about. He told me. So I mentioned how I was going to leave him if he decided to even smoke weed again. He told me I never said that and just gave me the craziest look. Well last night I walked into his music room while he was in there and I smelled weed. So today I go in there and I find his stash. He's been hiding it from me and he's lying. What should I do? 😭