Here's a suggestion. You can tell him that you care for him very much, but right now, a relationship isn't a good idea. He's got some things he needs to sort out, and you can't help him with those things. You can tell him you want him to recover, and that you believe he can do that, but it has to be without your help--he has to do it for himself, not to please you, and not with any hope for a specific future with you.
Tell him that when he's solidly sober, for a significant stretch of time (if he presses you, say a year) that he can give you a call if he chooses, and you can revisit the idea of a relationship if you're both in a position to give it a shot. I wouldn't hold out any more promise than that you will answer the phone if he calls after that length of time, and I WOULDN'T put my life on hold waiting to see what happens with him, either. You both might meet other people, and that might be what's meant to happen.
Just my two cents.