Openness, patience, and honesty are the keys. Try your hardest not to be defensive (and not to take anything too personally, if he gets defensive).
Since abuse screws up the ways we relate to people, it's very possible that he's facing up to issues that were just between him and his ex. Or he's asking questions about the time before you came into the picture.
It sounds like your biggest worry is about losing the man you love. I doubt that anyone could fault you for that position. Even if he doesn't feel comfortable repeating his conversation, he should at least be open with you about the fact that he IS talking to her (and how often); that is a simple courtesy to you, and the relationship you are in together. Talking to an ex does not automatically mean that he is trying to be unfaithful.
Hope this helps.
We're in this together. - Nine Inch Nails