My heart goes out to you, CT, I've been there, and I still sort of AM there.
I remember when my daughter was barely 2 and she was learning to speak, the firs time she said OW name. It wasn't clear,kind of her own version, but I knew what it was and I burst into tears. To this day, that is what my daughter calls her, and I still cringe.
My daughter used to talk about her all the time in a point where I hated her, and it killed me. Sometimes I would ask her to stop. Other times I would ignore, or answer through gritted teeth. But like Rose said, at 5, he is going to say because they don't have a filter, especially if it's something that is a part of their life and can't grasp why he can't talk about him.
My daughter is now 9, I and I have let go of my animonsity towards OW turned wife turned stepmother, I can deal. I am civil with the woman. But sometimes, when my D goes on and on about OW and her whole family, it gets under my skin. And NOW she is old enough for me to say "I'd rather not discuss it" and she could understand that boundary a little better.
For now, it's probably something you are just going to have to breathe through. Sounds dumb, but I would just pause, take some deep breaths, and change the subject.
I'm sorry you are hurting. It will lessen in time, but I really do feel ya.
