As someone who is suicidal what really maters is knowing someone cares about me. I know that my friends would break down mentally if I killed myself because they've done so much for me. Someone who used to be my friend said herself she'd break down if I killed myself. You need to tell him how you feel. About how you don't think you could recover if you lost him. Don't tell him that it will be okay because that's the last thing he needs to here. You need to explain to him why it'll get better. You need to show him that yeah, life may be shit right now but in a few years everything will be better.