Hi @utopia - I think I'm qualified to comment as I was once a 15 year old boy, came from a single parent home and had an alcoholic mother (now deceased). I also have one teenage daughter and another not far off so I can also relate.
OK for starters I'll tell you what I say when I get "dunno" - I will say "Dunno is not an answer" and keep asking until I get a proper response.
Remember you are his parent not his friend - you are the adult he is the child. I don't know whether your mother is undermining you or not - you both need to be on the same page. She has to tell him that he needs to come home. My kids love going to their grandmother's as it's all Netflix and junk food and getting away with murder. Your mother probably likes having the company so she enjoys having him there and maybe there is not a lot of discipline.
Probably correct in not taking him to a psychologist if he is not going to engage - perhaps there is a school counsellor or family friend he can speak to? If his father in his life or not? Mine died when I was very young so it was mostly just my mother but if he can speak with an older male role model that would be beneficial
As for your fantasies - forget it. You've come this far and made awesome progress. You have much to be proud of in yourself. You don't need alcohol - you've proven that. I drank a lot when I was younger before I had kids and even one drink makes me feel ill now, or maybe that's just old age? Either way it's not necessary and I don't think it's good role modelling.
Sorry if my responses are a bit harsh... keep up the good work and look after yourself. You can do this.