Such a simple question, and I just don't ask! For so long he mourned her (almost 10 months!) and was so angry at me for "breaking them up." Then, he just didn't want to think about it and hated it when I brought her up. So, I just didn't. I assumed he still thought "loving" thoughts about her.
So, I took your advice. An opportunity came up today when he got some mail from OW's hometown. He freaked out for a minute thinking it was from her attorney. (It wasn't) Their last communication was from her attorney threatening my WH with a law suit if he misappropriated any of her business ideas. It's a ridiculous notion, but it scares him nontheless. So, I used that as an opening to ask how thinking the letter was from her attorney made him feel. It made him nervous, but it also reminded him how incredibly immature and nasty OW is. I then asked how he felt when he thought of her Bday. He said, "Nothing". He said it crossed his mind just before he fell asleep, but it was just a passing thought. He didn't have any urge to speak with her, wish her well, or wonder what she was up to.
For us, that is progress. Thanks isadora.