I agree. You need to get out of this relationship. Please give up trying to make him understand. He doesn’t want to understand. He wants to hold onto you and he’s even willing to threaten suicide to make you stay. You are not responsible for his life. He is. But I understand why it scares you to think about walking away when he could harm or kill himself. For that reason, you need to turn his care over to someone else.
Whether or not he is close to his grandmother, she is still his family. Make a time to talk to her privately. Tell her that he is holding you hostage with a suicide threat but you are going to break up with him. Suggest to her that she be prepared to take him to an emergency room if he follows through on his threat to try to hurt himself. He may need to be hospitalized to keep him safe. If she can’t or won’t listen, you can talk with your school counselor about how to turn responsibility for his life over to professionals.
When you do break up with him, expect a tantrum or pleas or tears. It sounds to me like this fellow will do everything he can to manipulate you back into the relationship. You don’t need to give in to that. If you waver, he’ll figure he can always guilt you into staying. You can tell him how sorry you are that he feels so desperate but that you are not able to give him what he wants and needs. Calmly suggest he get into therapy and leave. Then make a call to whoever you talked to about taking care of him. He may still try self-harm, but if you make plans for his safety in advance, you can feel that you’ve done all you can.
I wish you well.