Sounds like you know what you want but where does he stand? If it's about getting his drinking under control than he has to be committed to it. As for myself, I run a lot, so at 4th of July parties if I've got people around me that won't respond to me saying, "I just don't want to drink" then I tell them I'm training for a race. I use that example to point out that he can pick out an excuse to get people off his back about the subject but if he feels like the pressure is going to be too much and you really don't want him to drink then he will have to walk away. I know these might not be the one size fits all answers for which you're looking but there is no easy answer.
Also, "I'll just have one" is famous last words that have been used by millions. If that statement has to be thought or said then it's not true. A person that can have just one doesn't have to tell themselves that. Anyway, Let him know how you feel and maybe you'll find it's important to him too.