Cutting down a child isn't about being drunk, it's about being mean. There are a lot of alcoholics who are kind to their children, even as their behavior creates chaos for the family. This is a problem separate from drinking too much.
Your son's condition sounds alarming. Does he have access to some neutral third party like a counsellor who can tell him (diplomatically) that his mother is behaving terribly to him?
This hits home a bit for me because my alcoholic ex-husband's mother was a mean alcoholic too. She cut down and criticized her children when she was drunk for their appearance, their interests and their emotions (or lack thereof) and they are all messes now, with addiction and/or mental illness manifesting in all four (the fifth one is dead). You can help your son escape from the cycle.