What does he need you for? To have someone he can shout at or threaten?
I could say, bluntly - though I know it's not that simple in 'real life' - that if you are not helping him get better, then you are keeping him the way he is.
I could say, again, bluntly (over bluntly), that you could make it clear to him that IF he wants you to invest any more of your life with him, he gets treatment for his addiction, and for his MH. Or you leave.
I know this sounds very blunt (!), but the dreadful truth of MH is that it is very, very easy for those who love them to slip into not 'supporting' them but 'enabling' them.
When you support someone you help them THROUGH their problem to a 'better place'. When you 'enable' someone you just let them stay where they are, and they never change.
If you read through some of the posts on mental health you will see time after time this difference is vital!
Do you think his MH problems started BEFORE he became a cannabis addict (ie, his 'root problem' is his MH, and he effectively tries to 'treat' himself for it by doping up on cannabis)? Or do you think it is his cannabis addcition that actually causes his MH? Again, important difference!
(Also important not just in what caused his MH, and how best to treat it, but in how much 'sympathy' one can have! Someone with 'innate' MH, so to speak, gets a larger sympathy vote than someone who brought it on themselves by indulging in an addiction....)(THAT SAID, I do appreciate that sometimes people become addicts because they have dreadful problems in their lives not of their own making, that they need to 'blank' with drugs and alcohol....)
SO, I guess, overall, I'm asking you what you think caused his MH in the first place, and what HE is doing about it (other than doping himself up!), and whether he is getting any treatment at all.
It does sound, from what you say, that he's getting worse.
Do you have children, because if so, that, again, can make a difference as to your priorities.
Kindest wishes, and I'm sorry to be blunt, but sometimes it's best to try and cut through to the heart of the matter. Few one ever posts first time on this forum without there being some kind of 'crisis' imminent! So it sounds like you are reaching the end of your tether with your husband.
Wishing you all the best possible, and a better future IS possible, even if it takes a lot of courage and effort to get there (his included!)