Hi OP. If you’ve been with your DP for 7 years that doesn’t sound to me like this boy is simply “her child”. You’ve known him since he was 10, lived with him, and it’s likely he sees you as a parent figure, however subconsciously. So my thoughts are in two parts:
I can see that these behaviours would be awful, but at 17 his behaviour is an outcome of local influences, yourself included. It doesn’t sound like your reaction to him is going to help parent his behaviour, as it’s quite clear he can summon a reaction from you even to the extent that you leave his home. The best response you can have from a parenting stand point is to be stable, be there, to find a way to not resent him, to have firm boundaries (eg if drugs show up, they get flushed) and to not give attention to what he’s doing. Engage him in family activities, do some bonding activities with him… it won’t be great at first, but then things have gotten out of control and this has happened on your (and his mum and dad’s) watch.
So that’s if you want to be a parent, and tbh I think as an adult on the scene anyway you shouldn’t be “resenting” or letting your feelings get the better of you. Children are always hard — it’s not quite the same thing but I know people who say things about their baby like “he’s a little monster, he will not let me sleep!” and I wonder if you’re that kind of person, to resent rather than look for solutions.
If you absolutely don’t want to be a family with your partner (because obviously she is a parent) then I think you need to cop to the fact that it’s not only his bad behaviour, it’s that you don’t want to handle it with her. Obviously your partner will always need to “handle” her son, and not having your support must be making things harder for her. In that case, living apart is fine. I just think it needs to be clear that you see your role as her partner but not her co-parent (so she can make a choice as to how she feels about that), and if you do move back in when he has left home, and he comes back to visit, I think you need to find a way not to get wound up, because it will only make things worse for your partner and this boy will always be her son.