Hurting

My boyfriend is hurting. I don't know what all is going on, he says he can't exactly explain it. I know it has something to do with not being able to live with his dad and being with his mom is tearing him apart. His mental health is decreasing so badly. I've been writing him notes, hugging and kissing him, saying positive and encouraging things. When he says something negative I always turn it to the positive side of things. Today he told me that he wants to be with me but what would happened if he couldn't hold on. He said what if he can't hold on because of his mental health. Last night he said he wasn't sure if we would make it and that he doesn't want me to make him anything or give him anything big, he said he doesn't want me to give him something with him not knowing if we will stay together or not. I've been encouraging him that I'm not going anywhere and that we will make it, that we will show everyone that we are ment to be. Hes turning 18 in a little less than 6 months now but he doesn't think he can do it. We love each other and only God knows how much. What can I do for him? How can I make him see that everything is going to be alright, that I'm not going anyhwere?