Your DH has had this under control for two years OP, and while I can understand why you're so upset that he's been drinking wine with his mum while she's over, it does show how little you trust him.
Once a year for a few nights isn't a serious or regular abuse of alcohol, and I wouldn't say snapping at someone is totally out of order behaviour. Neither of them mean his depression is back, but of course you're going to feel jumpy and have flashbacks to how it was when he was at rock bottom.
You've been a bit snappy yourself in your posts on this thread tbh (unusually for you (in a non-stalkerish way)), why do you think it's OK for you but not for him? Yes, we're only internet randoms, but hand on heart, are you sure that doesn't carry over into RL?
Whenever you're trying to cut something out of your life, food/fags/alcohol, it doesn't help when someone overreacts when you indulge in the bad old days. It can give you a sense of 'well I might as well be hung for a sheep as a lamb', giving you license to up the ante.
I'm not saying if he went back to how he was that it'd be your fault, but your reaction to this is important, and if you show your lack of trust in him, then he might as well carry on, (in secret even to avoid your control/disapproval).
It's plain you don't want to hear that, but there's no other way of saying it, and I'm sure he doesn't want feel he's not to be trusted.