I'm sorry for how you're feeling because I think we've all been there or at least something similar.
It sounds like you were some sort of rebound for him and that's really unfortunate, especially since your child is involved.
It is possible he didn't "sound like himself" or even sounded like he was in "turmoil." Obviously, he's probably in in some internally that he wasn't exposing (his fault, not yours), and now to let you go is probably not easy. Let's be real here -- even though people sometimes pick up a "rebound" and that is wrong, it doesn't mean they don't actually care for the rebound. I mean, they displace a lot of energy and love from an old person onto a new person, and through that, they do start to care. He probably did feel really badly about leaving you -- and especially your child that way. And the way these things tend to work (when exes pop back into the picture suddenly), it is like a light switch is suddenly going on -- there aren't always signs.
It doesn't mean he's a bad person, it doesn't mean he didn't really like or care for you or your son, he was clearly just still hung up on this person and he probably didn't even realize it himself completely until the moment he did. I think the best thing you can do is to give this situation space to allow you to heal. He's probably not someone you want to spend anymore time with because he wasn't solid enough in his stance with you to not be swayed in an instant. It doesn't make him a bad person, it just doesn't make him a reliable person for you or your family.
I'm really sorry, again, and good luck. :)