I sometimes find that my best defense in social situations isn't so much thinking up answers to questions that others may or may not ask me, but instead:-
Thinking of some questions to ask others (a change of subject is often a good way to deflect attention from That Question.)
Remind myself of all the reasons I quit (all those dark secret and shameful ones) and play the tape forward regarding what is likely to happen if I do decide to drink.
Get myself some secure exit strategies in place for the eventually that I need a break from the situation.
It may well be that you cannot shut him up. That's okay. His attitude won't reflect on you. It's nothing to do with you. Some people just behave like idiots at times. Sometimes dive to intent, other times due to ignorance. Let him bleat on if he wants to. There must be other people you can remove yourself and go chat to.
PS. If there are tables and seating involved, see if you can arrange to NOT be near that person.