You just moved, right? So you can't have the dog there?
Maybe it's time to consider finding a new home for her. Maybe that would make your son sad (if he's attached to the dog), but it doesn't sound like this is a great situation for any of you--including the dog. If the roommate kicks him out or something, what will happen to her?
Yeah, no doubt about it, he's a jerk at best. I'd just concentrate my efforts, if I were you, on making life as good as you can for you and your kiddo.