While I appreciate your caring and effort and concern this is NOT something that you can or should be helping with. This sounds serious and important and at 15 years old you trying to help may be dangerous — and not what he needs. Don’t get me wrong here. Your sensitivity is genuine and wonderful, but he is talking about killing people, a mother with suicidal tendencies and when he was invited to hurt you he did. As much as I appreciate your desire to help, it isn’t what either of you need.
Explain to your mom that you tried to talk and you think his needs are going to be much more than what you can do to help.