I'm NOT going to question your motives as others have, but I am going to agree with the general tone that you took the wrong path here. Your S doesn't need to know that his mom is a POS. Regardless of whatever else she is, she's still his mom.
Doing anything to harm that relationship (for whatever reason) isn't where you need to be. The most likely outcome of doing that is to damage YOUR relationship with him. He's an innocent bystander, here, and (IMHO) your role here is to make sure he's done the least amount of harm by this that is reasonably possible.
