At roughly the same age DD had a friend with a similar family background, with an absent juvenile (mentally) father and neurotic mother. She dropped out of school despite having a free scholarship at a highly academic local private school, where she was a high flyer. She left home and for a few months moved in with us before moving into a squat with her boyfriend.
Roll on 20 years, she came to her senses in her early 20s, got a degree, a doctorate and is now a university lecturer with a stable family.
No help in your current situation sturn22 but hang in there, just being there for your GS and an anchor of supportive stability in an unstable world. You may well find that in his early 20s, with the hormonal big dipper of adolescence over he may well get his life in order and your quiet supportive stability, despite the toll it is taking on you, will be well rewarded by a young man who settles down and appreciates all you have provided for him.