Maybe he's remembering happier times with her that he knows are never going to return. He's with you now. The person she used to be is gone. The person she was is never going to return to him. But, perhaps there used to be some happiness there. If I did what he's doing, I would be doing it to hold onto a cherished memory of something, perhaps to stop me from hating her, perhaps to prove to myself it WAS (not is) real.

I don't think it's about you in any way. In other words, he still remembers when their relationship was good. I have no doubt he knows that's not the case now. I also don't think it means anything more than that.

And I'm not sure it's even worth talking to him about.