I'd let him deal with his mother, unless you and his mom are so especially close that you'd normally confide in her. It's his problem to figure out, not yours. If it's to get "your version" out there first before he has a chance to trash you to his mom, that's a wasted effort. This is about your stepping AWAY from the madness, not smoothing things over.
The good news is that you aren't married. Whose name is the mortgage/lease in? Do you have any other financial entanglements?