Videotape thefts, pot smoking and throw him out. Get the police in to take him and his stash away if you have to.
You must have enough people in the family who wants to get rid of him to not let your mum back down and you can tell your mum there are resources available and he has friends so he won't be homeless for long.
I'll throw anyone out of my house if they start shit like that and it feels good when you know they're gone, too.