StayGolden, you are doing a great job trying to co-parent. It is a challenge when the other parent is not quite at the same level. It's like being cooperative in a vacuum.

Just want to ask--is the way she is talking to you as if she was still your sweetie? As if you two were still together and intimate or best friends? Or is she talking to you in an argumentative, hostile way? Depending on which it is could call for different approaches.

She loses her temper? Shifts the conversation from the son to other topics in her life or your life?

Can you guys work together to establish some ground rules for your interactions? Perhaps work to establish more of a "business" type interaction? Would having a neutral party help you do this make it more palatable to your ex-wife? Perhaps you could enlist the aid of a family therapist or a school psychologist, etc. Maybe if the recommendations come from an expert instead of her ex, she would be more apt to listen to them. A therapist could also help the two of you on your communication skills so your talks don't break down into arguments or get off topic. I agree that encouraging your son to act babyish sounds very odd, and I think a psychologist or therapist might be able to help her see this is not in your son's best interests.

I think it might be better to model the behavior of "never talking bad about his mom" rather than avowing it. It comes close to putting your son into a split loyalty situation, which is difficult for children of divorce. The son has now basically dissed his mom to you, saying she talks bad about you. It's just an uncomfortable role for one parent to be listening to a child diss the other. THe parent wants to be sympathetic and supportive, yet by being so, may be condoning the child's dissing behavior, which is often rooted in all sorts of stuff rather than completely in reality. The best response a parent can give when the child disses the other parent is to say, "I'm concerned about that. I will talk to your mother about it." That lets the child know his concerns are being heard and also that you are going straight to the source to verify things and figure out a way to fix the problem with the other parent. I know that might be hard in your situation, but it is a good way to respond to the child about complaints about the other parent. In my own situation (separated almost 1.5 years, getting close to divorce), there have been numerous times when our kids said something that the other parent did that was just not true, or they reported it out of context so it seemed worse than it was. Whenever my ex-H or I hear anything like that, we check with the other parent. This is also very reassuring to the kid as it lets them know that mom and dad still work together on parenting decisions.

I am recommending a book about co-parenting after divorce. It talks about the "business" model of interacting and has all sorts of practical suggestions. Although it has chapters on some of the early phases of divorce, there is still a lot that may be helpful. And if you like the book, you could recommend key parts of it to your your ex-wife.
Mom's House, Dad's House: Making Two Homes for your Child
This book, as well as a couple of others, were helpful to my ex-H and I when we were doing the split. It also helps establish some positive goals for the future regarding co-parenting and so I could see that after divorce, the parental interactions didn't have to be awful, hateful, destructive, etc. (the stereotype). We also had the help of a family therapist who helped us work through the rough spots and helped us establish as our primary goal, the well-being of our kids.

I disagree with this. You do not want to put your son "in the middle" of conversations you should be having with your ex-wife. It's really uncomfortable for the child. If you need to ask your ex-wife something, then do it directly. Don't use your son as the message carrier.

Keep at it, StayGolden. It sounds like you are doing an amazing job.