This is tough situation for you sorry you have to deal with that asshole. That is all he is. Sure he can be kind one minute but the next you just never know. I know people can change but it doesn't sound like he will even if he puts on a good show for the family. When he is alone with your mom that is when he will revert back to abuse. I can just tell that's what is going on.
Got to protect your mom somehow.