You know Mom drinks too much ... Mom knows Mom drinks too much ... your son knows there's something wrong with Mom and probably suspects she drinks too much ... so why not acknowledge what everybody knows: "Mom was a little off last night, I think she drank too much alcohol". You don't have to get into judgment of Mom and you don't have to get into details - just validating your son's perception that there's something weird.
I am concerned that you're heading into elephant-in-the-living-room territory where everyone talks around the topic and tries to figure out who knows what, who will admit to what. etc. Mom-drinking-too-much is going to be a part of your son's life for a long time - maybe now is the time to start being just factual about it?