I think that you are doing the best you can to stand up to this BS, with incredible odds stacked against you.
Your husband is wrong. You are strong, but I can feel your frustration like you are banging your head against a brick wall. Your husband seems clueless.
20 years old and drinking 3 cases of beer a week is a horrible start to his adulthood. He is manipulating you and his girlfriend. Don't be surprised if she dumps him soon, it sounds like he is making her miserable.
This is more than college drinking and partying, he is self-destructing in front of your eyes and as a caring mom, it is very hard to watch.
I hope you feel better today and you are ok.