My Boyfriend has become stoner... Wtf should I do?

So I've been with my boyfriend for 7 months after been single (out of choice - due to a previous head screwing relationship) so I give this guy a chance and jump in both feet first.

He smokes weed but then I drink too much initially so we get on quite well and he just finds me funny...

He is "quitting weed" when I met him so I cut down the drink as I only do it cus I'm lonely... He then stops completely... 3 months of pure happiness and he would move mountains to come see me and spend time with me, she's I'm his de-stress... He's working overtime to pay for Xmas for his family in replacement for what his father should be doing... I'm a proud bf having someone who is so thoughtful and I think the world of him...

Jan comes and he start smoking occasionally which is fair enough... Then it starts becoming everyday... Suddenly he stops coming to see me, if I want to see him I have to go to him, the calls stop, the texts become one word answers, I write him a letter saying I'm leaving him as this isn't what I signed up for... He ses he's going to quit rather than loose me... He carries on... February I take him.to New York for valentines day although at this point I don't even want to be around him it's gotten that bad... He's not smoking for a week & I get my boy back & fall for him all over again... We get back to the UK and he meets up with all his mates, within 4 hours of being back he's got a spiff in his hand... I then start a new job the next day and I'm on a course for 2 weeks... He's smoking heavy now while I'm there, no real contact unless I call or FaceTime...

I keep leaving him and asking to come to some compromise that he doesn't smoke only on days I'm with him out of respect... So the first thing he does is get stoned before I get there or burn one as soon as I arrive...

He's decided he don't want to work more than his 13 hours a week, which don't even pay his bills, I've been subsidising him for months, I've split up with him loads of times to the point where I have given him the ultimatum of "me or bud" and he has said he's chosen me but then carries on exactly the same... I'm seeing random guys messaging him and calling him, he deletes everything off his phone & I'm trying to work out if it's the bud or that he don't wanna be with me...

I've told him loads of times that if he don't want to be with me to just say but he says he's happy with me, I've broke up with him for weeks and he just carries on like we're still together with the texts, but still no effort is made in coming to see me, calling me, anything I've asked he tells me what I want to hear at the time then just does whatever the hell he likes... Is this a common issue with bud smokers?

I keep being told I'm being over the top and that I need to chill but I really think he has a massive problem and it's going to ruin his life, I don't care if I'm with him or not I'm just concerned cus I do have feelings for him that he's going to get to 30 and still be doing what he did when he was at school... ALL of his friends smoke it & when he quit for 3 months before he didn't see much of them at all, now he would sooner spend every waking second (when he is awake) with them than me... He tells me there's nothing wrong with him and that he's fine, I'm just been over the top and doesn't see what the problem is, he has smoked it since he was 12 and he's now 26 so that's a long time...

I don't know whether to just walk away or how to get someone to see that they are wasting their life and they have a problem which changes their whole personality and is ruining a decent relationship, when they clearly have no intention of changing...

Thoughts?