How to Deal with a Mama's Boy

Is she really stepping over her bounds or is he having her do his part so he doesn't have to deal with you and the ensuing problems? I have seen both side of this fence. Does he live with her and te childen stay there and she cares for them? Then ou want to have her on your side and not as an enemy as she may feel much more awkward than you know.
When it comes to communication that is always hard even in a good marriage so think back to how you solved problems when you were first together and try seeing if that helps. For one family member they only send notes so there is a paper trail and anoter has a in between person that helps. Good Luck

My ex's mother used to try to step in. At first she acted like a mediator between us, but then it got to the point where the ex would just have her call if he was running late or whatever. I just quit answering her calls.

If she continues, I would quit taking her calls, or, if you're more direct, let her know that this is between you and him and she needs to butt out.

I'm with MandA M.
Next time she calls, tell her it's "nunya." (None ya business!)

How good is your relationship with his mother in general? If at all possible, you need to try to get her on your side. This might mean a friendly sit down chat with her about how you feel and how her stepping in affects your relationship with your ex and that it affects your relationship with her as well.
When someone says a guy is a "mama's boy" I have a couple of different scenarios going through my head. One is that he is a wimpy sort of person who can't do anything without the approval of an overbearing mother. On the other hand, sometimes it's just a nice relationship in which the guy has real respect and love for his mother, but it sometimes gets out of hand when it comes to his other relationships. If your ex's situation is like my second scenario, I think you should be able to talk this out with each of them and get the kind of results that will help you communicate better. If it's the first situation, I'm not sure how well it's going to work.

She actually steps in to talk to you? Really? Wow.
I think I'd point blank tell her that it's none of her business, and that this is between you and your ex.
I have NO patience for that kind of thing!