Honestly, COD, I think it's more important that you figure out a way to talk to DS about what happened last night, instead of expecting it to be some great revelation for her. She knows she is a mess; she is not ready to deal with it yet (if she ever is).
Meanwhile, he notices. He sees. He knows it isn't right. In lieu of an explanation, he will invent one of his own. And because he's just a kid, it will most likely involve it being a) his fault, and b) his responsibility to fix somehow. I know because this kid was me. "Tired" isn't going to hold up for long.
It sounds like there is a tremendous amount of resentment in your household from all directions. I am sure you believe you are protecting your son from her addiction, but there is alot more going on there than just her drinking, and he is present for all of it, even if it doesn't seem like he is.