Honestly? I'd let him know now. It won't hurt any less to tell him in person. it doesn't seem fair to string him along like everything's wonderful. If he drinks, he drinks. Life is full of disappointments, and one of the challenges in learning to live sober is to deal with it without picking up a drink. If he chooses to use it, he's got plenty of support to lean on. If he chooses not to use it, well, he's probably just not ready to quit.