He wants to save the marriage? Show him the new rule book

So he's admitted his affair. And is assuring you it's over. And that, he's telling you, is all you need to know. Doesn't matter that you're asking for more information. Doesn't matter that he's clearly lied to you in the past. That's all behind him now. And it's for your own good, he insists. It'll hurt you to know more, he says.
Problem is, the rules have changed. And he needs to know that. He's no longer the one determining what will hurt you, or what you need to know. I clearly recall my then-5-year-old daughter telling her little brother, "I'm the one in charge here." And guess what! You're now the one in charge here.

You decide what affair details you need
Not to suggest you be dictatorial...well, actually, that's exactly what I'm suggesting. We trusted our spouses and they let us down. Blind trust is gone, never to be seen again in these parts. Mature trust – trust that is earned through your spouse repeatedly doing and saying what he honestly did and said – can bloom. But not until you get the answers you need and he starts giving them to you.
The thing is, cheaters feel lousy. Unless they're psychopaths, they know what they did was hurtful and selfish. And by giving you the details of that, they're forced to acknowledge that not only to you (who kinda already knows) but to themselves. It's a lot easier to stay mum, and pretend it's because they don't want to hurt you. They already have! But to face that hurt in your eyes again and again feels crappy.
Unfortunately, I don't know of any couples who have successfully put their marriage back together without this brutally painful step. As one wise soul said, during her husband's affair, the door was shut to her. She didn't know about it, couldn't ask about it. Now's the time, she said, to open the windows to her and shut the door to the affair partner.

Don't go "pain shopping"
I'll add in a caveat, however. There is such a thing as too much information. Some call it "pain shopping" and it's a compulsive need for more and more detail. I was certainly guilty of it in the early days following disclosure.
The danger is you can't "un-know" something, no matter how painful. After a while (I'm a slow learner!), I came up with my 24-hour rule. If I wanted to ask my husband something about his infidelity, I made myself wait 24 hours (give or take...). If I STILL wanted to know a day later, or could even remember what the question was, I proceeded and he answered honestly.
Most times, the question had faded from my mind...indicating to me that it wasn't something I needed to know but rather "pain shopping."
To my husband's credit (I do give him some, now and again :) ), he recognized that I deserved to decide what information I wanted.

The new rules?
You decide what information you want.
You demand total transparency going forward. (More to come on this in an upcoming post.)
His job is to support you as much as possible in your healing. And, believe me, it's not a job for the faint of heart.