When he talks about the other woman, is it in response to you asking questions about her? If so, he is probably afraid to show any emotion (doesn't want to hurt you) and hopefully a little ashamed/embarrassed to talk about her with you since he is admitting to breaking your marriage vows each time he talks about her.
In reading your other post re: your depression, what I would gather is that he still shows loving signs towards you but is afraid you will not really change in the long-run. I'm glad you are moving in a positive direction for yourself and pray he will take notice.
The more I hear about spouses that stray, I'm starting to think they have this pollyanna view that this new-found relationship is everything they were missing, but in fact it seems many had this in their marriage, it just needs to be brought back to the forefront.
Logically, the other person is not perfect and if they choose to run to that, eventually their issues will surface and the spouse will be in the same boat. Why not work on it with the one you made your vows too and have children with? I guess being caught up in all the attention from someone new makes you throw logic out the window.
Sorry, preaching to the choir but I hope he comes to his senses for the sake of you and your son.