What do you know about his disorders? How old is he? It seems like this is a really hard situation and there are no easy solutions. My heart aches for you because I have some small idea of how hard this is to cope with.
Have you considered residential placement for him? Is he planning to go to college? How is he planning to pay for it? The expulsion will cause problems with that. If he has Reactive Attachment Disorder (RAD), how can they expel him? Doesn't he have an IEP with Reactive Attachment Disorder (RAD) on it? Isn't threatening to kill others a manifestation of his disability/disorder/disease? How can he be expelled for that? I don't think that is legal, although I don't know if you really want to fight that battle right now. I know with my son, I chose not to fight some battles because I did not want to send the message that he could get out of punishment for hurting people because he had a disorder. (My oldest has Aspergers and Obsessive Compulsive Disorder (OCD) and some other things. He spent years trying to kill his little sister. He knew he had to kill me to get to her, so his plan was to murder me too. He was perfectly happy to leave his Dad and little brother out of his violence). This wasn't little kid stuff. It was serious attempts to murder us. There were times he got consequences for things in school and during his stay in the psychiatric hospital that I could have fought. I refused. I even told my son and his teachers that I would not fight the consequences because he needed to learn that violence has consequences.
I don't know how well your son can learn these lessons. My son COULD learn them, he just didn't want to. Your son may or may not be capable of learning them. You would have a much better idea of this than I would. I don't know if you want to fight that battle right now.
Has your son dropped out of school as he cannot attend? What is his plan for the rest of his life? How do you see him fitting in with the rest of the family as an adult? I am assuming he is close to 18 years old, so he is almost an adult. You may have to decide if he can continue to live with you or if he needs to leave. You will have to figure out how that will look. What will he need to do to support himself? How much, if any, support are you willing and able to give him? How much, if any, assistance are you willing to give, and is he willing to take, once he is out of the home? I do recommend NOT NOT NOT cosigning for ANYTHING, EVER!!! Either buy it outright, or let him buy it with his own credit or lack of. Don't put your credit on the line for him to make payments. It just won't be something that works out positively. It will strain your relationship to the extreme.
This is hard, but maybe he cannot live with you any longer. Maybe he doesn't even want to. Safety of the entire family MUST be a high priority. I hope you can navigate through this. If I can help in other ways, just ask.
(((((hugs)))))