It's all very crystal ball, but I think you have to be cruel to be kind here.
I do think that things may slip back to where they were, it sounds like, essentially, aside from where he lives, things haven't really changed for him - he is still smoking weed, went to, but didn't commit to counselling.
I also think that, no matter what your wishes are for someone, unless they are willing to seek help, and get better themselves, it's a lost cause (I'm not saying he is a lost cause, I hasten to add, but just saying try not to rose tint it all)
I daresay, his weed smoking is him 'self-medicating' and masking an underlying condition - his behaviour is becoming 'self-fulfilling' - it may be depression, it may be something else, whatever it is, nobody will know until HE wants to go and seek help, and I wouldn't envisage his behaviour altering much until this point.
Kudos to your Son, he has a job, so he has responsibility, he has accomodation, this is responsibility, if he comes back to living with you, it takes some of his responsibility away, both in financial terms, and practically. It also lessens the responsibility of his job in a way, from a psychological perspective at least, because right now, he needs his job to pay for his accomodation, the onus, and responsibility is on him. If he moves back in with you, then this responsibility may not sit so heavily on his shoulders because, regardless of work, he will always have a roof over his head.
I am a Mum myself, to two very little DS, but I am speaking from the perspective of a sibling.
My sibling 'ping-ponged' back and forth to my parents house for YEARS, self-medicated, manipulated, worried them sick - it was only after years of such behaviour, and him hitting absolute rock bottom, and saying HE needed to go and get help (after, literally, years of us all saying the same thing) that he was finally diagnosed as having bi-polar - he is in a completely and utterly different place now, a very positive one, we still never hedge our bets, but the change in him is amazing, and I've gone from a place where I was almost 'done' with him and his behaviour, to being able to say I'm proud of him.
I know it's different for you as a Mum, and whilst I'd like to think that due to my experience as a sibling, I would do things differently if it were my children, I know it's a very different perspective - but just giving you some insight and another perspective on things.
Stay strong, and also, like I admire how my parents have been there for my sibling, I just want to say how lucky he is that you continue to be there for him. (And you ARE still being there for him, and NOT letting him down in an way, even if you don't let him move back in.)