He knows what he's done, but he's buried it deep down.
It suits him to believe that you weren't bothered, because then he can live with his cruel and selfish decision. He can look people in the eye and say, "AleC and I were unhappy, we had grown apart'. By doing this he feels justified. It's weak and selfish, but my ex, and countless others I'm sure, all pedaled this lie, to make themselves feel better.
The trouble is, everyone knows it's a big fat lie. As does he. So, seeing you coping, strong, putting on a front and making the best of it, reinforces what deep down he knows: he walked away from a strong wonderful woman and everyone thinks he's lost his mind., He let you and the kids down. The fault is his and his alone. So he's quiet and withdrawn, because what he's done is try to minimise the devastation he's caused but it can't be buried, it's huge.
A good man would have told you a long time ago that he was unhappy and would have moved heaven and earth, tried everything to make his marriage work. But he didn't. My ex didn't either.
Everyone has the right to walk away if they no longer love their partner. But when kids are involved, they must try to fix that marriage. If you both had the chance to do this, if he had been open and you knew how he felt, that would have given him, you and the kids every chance to keep your family together.
He chose instead to walk away, rather than fix it. Now, on the outside he can see only too well what he's walked away from. His lie, his premise that you both wanted the split only works if everyone buys into it. You never did, so he's left with a lot of thinking to do. If he has a shred of decency, he will 'own' his selfish decision, and do all he can to build a strong relationship with his kids. He will respect your anger.
Unfortunately the sort of man who will put his kids first now, is the man who would have been honest months ago, and tried to work with you to fix your marriage.
He's physically gone, he's now facing the truth that mentally, he can't just wipe out the past.