There is a saying that goes something like:
Someone may not remember the things you said, or been able to see the things you've done, but they will always, always remember how you made them feel.
Speaking from someone who's been in a relationship where the other person cheated (every single relationship up to my current one..yes I know how to pick them) it is very very difficult to just "move on," and even years later, now that I'm with someone else, those things still haunt me.
Communication is the most importnant aspect here, I think. Bringing it up over and over again may be his way of getting it out of his system. Maybe talk to him about things you can do to open up the trust and communication between you two (access to emails, cell phones, etc) to help rebuild that trust and help him feel secure again.
Best of luck to you.