i'm not discounting your description of situation. but it's at least possible this has nothing to do with what he's heard from "her".
how often do we tell our children to go to bed and stay there? that doesn't make us bad parents. we usually forget to give them the list of reasons we hope they'd get up anyway when it's really the best thing to do.
praise him for being a good son and "citizen of the household" and then tell him it's ok to get up if he needs to visit the toidy, to let you know he's sick, etc.
grownups are as confusing to kids as they can be to us. it may be that a little clarity is the only thing needed in this situation. ;-)