OP, you don't appear to have made any comment as to how your husband feels about all this, why does he appear not to be involved?
What is your husband's opinion on your son's behaviour?
Have you told him that he's not allowed to do anything to sort him out because he's not your son's father?
Has he said he doesn't feel he should step in because he's not your son's father?
Or, has your son said 'you can't tell. me what to do, you're not my Dad?
The fact is, that your husband is the only person physically strong enough (presumably) to deal with your son, I'm not advocating violence, but boys of that age are quite often bigger than their mothers, and feel able to intimidate a woman in a way that they cannot with another man.
In my opinion, you need to speak to your husband, tell him that you want him to act as a father figure to your son, and that you want to work out together the best way to handle him while he is in your care. You should make the rules between you, so that your husband feels involved, and in my opinion the rules should NOT allow your son to vape or smoke weed. Your husband should deal with the age old 'you're not my Dad, you can't tell me what to do', by saying 'no, I'm not your Dad, but I am your Mother's husband, and while you're living in our home, you will do what your Mother and I tell you to do.
Has your husband ever sat down with your son and really talked to him, or does he just leave it all to you, because, a) He thinks he's your son, so your problem?b) Because he's another lazy father who can't be arsed to parent a child even though he's willing to give him a roof over his head? c) He's afraid that his father will seek him out and be violent toward him because he doesn't think he should have any say in how HIS son is raised? Or d) some other reason that I haven't thought of?
Unfortunately I had a traumatised teenage to deal with after a marriage break up, and the difficulties that followed, but that was years ago, when there really was no help for parents, and you had to find your own way through. I made a real mess of it, and have many regrets, which is why I feel for you. However, these days there is a lot of advice available to you by way of the internet, if you're prepared to read it, and fully immerse yourself in trying to help your son. For example, have you looked at this: https://www.nhs.uk/mental-health/children-and-young-adults/advice-for-parents/teen-aggression-and-arguments/
Obviously having made my own mistakes I know that it's not as simple as reading a few lines of text, and all will be well, but if you truly love your son, and want to help him become a decent man, then you can't just abandon him to his father.
I think that one of the things that I would determine, is where he actually wants to live, and then tell him that he either lives with you on a full time basis, or he lives with his father, but there is no more of this a few days here and a few days there, and once he's made his decision he has to stick with it. Obviously he can still visit the other parent but there are to be no overnights, and he cannot expect the parent he choses not to live with, to keep a room for his sole use. So if he choses to live with you, and runs off to his Dads, then you will no longer be providing a roof over his head, and he will be stuck with his Dad until he is financially capable of paying for his own home, BUT here is the important bit, you have to MEAN IT, and STICK WITH IT yourself.
If he choses to live with you, then you should go ahead and start work on building the extension you mentioned, to show him that you are committed to him living with you, and that you WANT him with you. He definitely needs a place of his own at your house if he choses to live with you.
Then commit to doing whatever it takes to help him sort his life out.