R'd at 2 2years, and now 5 years out here. So this is my experience with this....
I told him all the time post Dday (first few months) that I needed reassurance frequently, but couldn't be more specific as I was so wounded I just wanted the bleeding to stop.
A little further out he was great about the ILY's, but would not bring up the A voluntarily, and that was not fair to me. I would ask him, do you think about it ever? Do you have feelings for her still? I would get the answers that were of course Not really, no. but not much more.
What I came to decide, and do as I continued to heal myself and get stronger, what I really wanted from him was the reassurance that he was happy he stayed, that he wanted to be with me, and the kids. I also realized he was a bit of an emotional retard. So I would have to be very specific with him on what I needed.
I would tell him I need to know at least on a weekly basis that you are happy with me, with the choice you made, and that you are doing everything you can to help me heal. You can choose how to do this anyway you want, by giving me a card, by saying some heartfelt words along those lines, by giving me a little extra attention, but so I know that you get it, and that you are here for me.
For me his actions spoke much louder than his words, and as we came to about the 5-6 month mark he was really really getting it, and doing what I needed. He would do silly things like leave candy on my car while I was working, send me an email out of the blue with some sweet message, that he would do some extra chore around the house that I dread. All of those things communicated to me the assurance that he was there to stay, because he wanted to be. Not because he had to be.
I hope this rambling makes some sense. But it is important for him to know that you need the assurance and that he is all in.