Full disclosure in every area where he could possibly deceive you by contacting any OW.
That's a must.
A certain amount of time, maybe a couple times a week, or a certain amount of time in a day, where you are allowed to speak freely about your feelings, fears, concerns, frustrations, etc.
That's a must.
Acknowledgements from you that he is staying true to his word.
That's a must.
Refusing to hold the past over his head in disagreements, either in word in your attitude.
That's a must.
There's give and take on both sides. YOu are interpreting him as non-chalant right now. YOu just need to consider that this may simply be his way of dealing with the competing guilt and feelings for the other person he has inside. It's not easy walking around each day feeling like a jerk.
You're both going to have to do things that are difficult and perhaps unpleasant. He doesn't even necessarily shoulder more of the load than you will.
No matter how you do it, it's not going to work if you grade his responses against how YOU would respond.
Blessings,
Bill
"Don't tell me the sky is the limit when there are footprints on the moon."