Maybe something to consider doing would be to anonymously get the police involved when you know he has drugs in his possession. Or even better yet, if he is not insured under your grandparents on the vehicle, report it stolen when he takes it (unfortunately, this will have to be done by grandma/grandpa as they are the owner). Or two birds, one stone: grand theft auto AND drug possession when he takes your grandma's van to pick up his drugs. Kind of an ignorant thing to do to a 'family member' (and I use that term lightly in this situation), but maybe a criminal record and probation may help improve his attitude. And if not, multiple offences ending with a stint in prison may bring him to realize the errors of his ways. Even though the Canadian justice system is extremely flawed, but that is a discussion for another time..
Or you can always just leave status quo as is, and move out. But this doesn't really help your grandparents with the situation they are in.