Hey LL
I don't think it means he is keeping the door open for her. She has moved on so it is pretty reasonable to think it is completely over.
I am still friends with my ex's and often catch up with one in particular but it doesn't mean I want to be with him romantically. He is more like a brother to me actually.
It sounds like the guy you're seeing is mourning that period of his life overall not just the women that was in it at the time. And that would ring alarm bells for me because it means he hasn't moved on from his past and is not living and enjoying the present moment. We often idealise the past and then think we need to find the same thing to be happy again but it isn't the case.
I suppose at this point you can't really bring it up with him but if it turns into something serious maybe point out to him that he seems to talk about her a lot and see what he says. I wouldn't worry too much though.
Some people are drains and some are radiators... Keep clear of the drains and hug the radiators!