Why not be honest with him? If you separated because your drinking was out of control, and he's not much of a drinker himself, then he would surely be pleased to find you sober?
I don't think you have to go into any detail, but just say you've decided to stop as it was making you ill?
It is a huge deal for you but normal drinkers don't see alcohol like we do. I think you may be pleasantly surprised that it is less of an issue for him than it is for you xxx