MROO, I'm not in the same place as your kid but I went through a relatable list of BS from my own family. I have some serious history there I won't lay on you right now.
The best possible thing you can do for this kid is be on his side and do everything you can to make sure you're there for him. And if he reaches out to you, do your best to find professional consistent help while trying to respect boundaries and give him time to process. And most importantly, listen to what he says and believe him - not because you don't like the parents but because he said he lived through it.
I can't even BEGIN to tell you how valuable even one person in my family being there and being supportive towards me would have been. If he comes around, it matters a lot that you're there to listen and support. i really hope he does for you.
Also, with regards to suicidal thoughts... It's important you don't just out people for the reasons you've experienced... When suicide prevention hotlines get calls they're rarely responding with "Get help don't do it go tell the cops/whatever". They instead just sit on the line and talk to someone. They don't try to talk them out of it - they just listen and talk to them. They do everything to believe them and make them feel cared for or slightly normal. The reality is hard - if someone is going to try to kill themselves the only way to stop it is to have someone physically stop them which can be traumatizing in and of itself and create fear of speaking out or reaching out for help. So professionals know that the best possible way to get someone away from that metaphorical ledge is to make a space where it's safe for them to talk about it and feel heard. Make sure you exercise caution around it, and just make sure he knows he can come to you if he needs without being traumatized all over again.
I have WAY too much direct experience with this stuff unfortunately... I hope it ends up working out for everyone in your family in the long run.

A second family is worth more than anyone will know.