This is terrible -- I'm so sorry to hear about it. I agree with you that all of his previous actions point to his still having feelings for her, as painful as it might be for you to hear, I think deep down you know this is probably likely. I don't think that means he doesn't have feelings for you, too, obviously, and it is possible to have feelings for more than one person, especially in his situation with their past history. I think he is just very confused and doesn't know how to handle his emotions -- like you said, people should (in ideal circumstances) be able to separate themselves physically and emotionally when they move on, but for whatever the reason, this does not seem to be the case for him and his ex. It's a very unhealthy situation for him, but also for you, because the whole thing just isn't fair to you at all. He can't have it both ways -- it's either you or her, and the fact that they have a history and a child together doesn't excuse his behavior. I know you probably care for him very much but I can't help but feel that it would be in your best interest to either tell him straight out that this behavior must stop (if you still feel like you want to be with him, that is) or that you need to leave him, for the sake of your own mental and emotional health. You deserve to be respected and right now, he's not doing that. I hope it works out for you, and I wish you the best of luck.
You're welcome. Unfortunately he seems to still have issues he needs to work out with her. I think he might eventually be able to move on but he doesn't seem to be at that point yet :(