When he is sober, apparently he is a decent man or your mom wouldn't tolerate his behavior. If he doesnt get help this time, then next time: Video his drunken stupor and when he sobers up, have an intervention and force him to watch it. If that doesn't influence him to seek help, then the decision rests with your mom on her next course of action.

No one can force him into rehab until he is ready, otherwise it is a waste of time. money and won't last.