He's really lucky to have you as an uncle @seesaw . This reminds me of some stuff I went through with my step son. I was lucky that I could spend lots of time with him, and help him get some strength and independence. I don't know if this is good advice, just my initial thoughts, but I would really support all his ideas about the adults he doesn't trust, and although he's very young, I would support his distancing from them, and his self care.
It is not good to have these levels of distrust, and have people scared of you. I was volatile as a teenager, and it really scared my mum, but she wasn't someone that could help me, and she was actually someone that really drained me. She thought she needed to make my life easier, and pressured me into things and out of things. It just wasn't right. My life was only better for distancing, and getting independence from her.
Of course he needs his parents at 15, but he doesn't need to like them. If they're doing things that are deeply hurting him, and giving him all this anger and despair, he needs some outlet for that. Are there things he does enjoy doing? Any things that put him in the moment, and channel his own ideas or passions? I think all the encouragement, and validation you can give will be well worth giving. It's really sad things are so messed up at home for him