I haven't been in this thread for a long time and I don't know what the etiquette of jumping back in is, but if this kid is still under 18 there should be child welfare authorities who could jump in.
If he's close to 18 and there's some safe alternative place for him to live -- like those paternal grandparents -- he could apply to be emancipated.
Been that route. PA does NOT have an emancipation program. The only way he could do it is if he can prove that he can hold down a job and pay his bills. Mom has not allowed either - no jobs last long enough without her ending them and he has not been allowed to get a drivers license. I sent the suicide text to both child protective services and the state police - Mom convinced both that I was over-reacting and he was just angry with them for grounding from his minibike. HE got told off by the police over it! The Kid also lied to his Mother about the text (before I sent it, I'd just told her about it) by saying that MY DD was the one who was talking about it, not him - and that his personal reference to it was from a long time ago.
If you have reason to believe he's in danger from them or from himself provide that information to the proper authorities. And if you know how to contact the paternal grandparents, maybe they'd be allies on their grandson's behalf.
I did. Through both my job with the public school system and my volunteer training (15 years as a BSA leader, 9 with GSA and 5 with 4H) I am a Mandatory Reporter - that means that I am required - BY LAW - as well as by my conscience - to report any suspected child abuse and/or neglect under penalty of fines and serious jail time. Even without the mandate, I would have reported this mess.
Paternal grandparents are not an option. She has a medical marijuana card and the Kid's Mom has convinced the authorities that she "shares" with children. She does not - it's a control and revenge issue. Grandfather is a recovering alcoholic who "Mom" has convinced the same authorities is still drinking. How she knows this when she doesn't have any contact with them is beyond comprehension - but she's a very convincing liar.
The grandparents are a resource, though, because they have access to the lawyer who handled the death benefits and trust set-up.