First should come the safety of your other kids. Then find the right diagnosis. If you can do both at the same time great, but right now just keeping them safe is the main priority. What kind if services/agencies are involved right now? Do you have alarms on doors and windows? Do you lock the other kids in at night and sleep with the key? Have you been calling the police or his therapist or case worker when he threatens you? Do you keep a behavior log? Are the other kids in therapy? How is he in school behavior and academically? Are there supportive family members the other kids can go to while you find a placement for your son?
I don't have a 15 yr old. When I sent my son to an Residential Treatment Center (RTC) he was 10. At the time he had a history of trying to murder his sibs and I could no longer keep them safe from him. He would hurt them right in front of me and was so fast I couldn't stop him. I really hope we don't have to go there again.
Lastly, welcome. And I am so sorry you fit right in here.