Obviously, I can't say what his feelings for her are, but it's not uncommon for people to simply fall out of love but still care for each other (which is probably what happened with him and his ex).
It would be so much easier to handle, I'm sure, if he had only been friends with her and not married to her - you could probably accept that he has a female friend without their being any deeper emotions there.
At the end of the day, he's with you, and if he's given you no reason to doubt his feelings for you then you should accept that he is simply still friends with his ex. In fact, it takes quite a man to be so grown up about relationships - so you really may have struck lucky here!
Having children together obviously gave them a reason to stay in touch and it sounds like what they have now is a good friendship and nothing more. If you intend to be with him long term it might well be good to get to know his ex a bit better - it would stop any awkwardness and make you feel a bit more secure in yourself.