I wouldn't mention it at all then. The only person you're accountable to is yourself. You aren't making any promises to him at all. I know you have in the past, we've all done that.
Just keep it as a non issue. If he offers you a drink, say no thanks. If he asks why, say you aren't drinking today because you've been ill. You aren't lying. If at some point you could go into more detail but only of you want to. I've been in drinking situations with people who don't know I've quit. I've said the same thing and the questions go away.
Wishing you the best of luck xx