so you seem hung up on the PA thing... will it make a difference in the long run if he is willing to be transparent if he did or did not have one? are you expected yourself to react one way or the other?
I had affairs in my marriage, and my wife wanted ALL the details. even though she wanted all the details now when we are doing something now and again it reminds her of what i have told her. while i think its healthy, its a fine line. if you truly need to know so you can move on then state it as such, but dont push him to be defensive. if he is truly trying to work on things with you, he might think it best you not be hurt more by knowing everything.