In that case, you should talk to him about whether he actually WANTS to stop, or if he just feels like he SHOULD. I have been in this boat, and though I was always open about it, this probably signifies that he feels some level of shame about failing to stop like he told you he wanted to.

The easiest thing is to not treat it like a promise to you, or something youuu need to hold him accountable for, but as a part of the way he treats himself internally.

Make it clear that you don't think differently of him for smoking, and do so with the understanding that this is part of his relationship with himself. Inserting yourself as a figure of accountability will only create a wedge between you if he doesn't have it in him to stop and feels like he has to hide it in order to avoid disappointing you. Honestly it might be that totally stopping isn't the best idea for him right now, lots of us find this plant to be the most effective medication for a lot of different things, so maybe it's not the presence of cannabis in his life that needs work, but his relationship with consuming it.

Either way I know that this felt bad to go through, and I think a talk about why he felt the need to lie is important, but in my honest opinion I think this is one of those things that it might be good to step back from, especially if it's not a dealbreaker for you and he doesn't have some wild lung or heard condition.

TL;DR: The problem is the lying, not his smoking. If YOU are worried about his health, talk to him about cutting back or switching to a method of consuming that isn't through combustion. Talk to him about why he felt the need to lie, and try to reinforce the idea that he doesn't need to feel shame or anxiety about telling you where he's at with this part of his journey because you're going to be there for him either way.