Poor bloke eh? Totally fucked!
Has a fling. Regrets it - grass is very much not greener alas, but too late now. Bugger. Tries to get you back. But even the ace card - 'I'll take the kids!' doesn't work, because you're neither desperate not thick (sadly, unlike his new lady love, it seems...)
No choice but to brazen it out. Translated into toddler speak - 'Don't care! Don't WANT the lovely chocolate cake anyway! Prefer the dry biscuit that I picked up by mistake, yes I do! Ner ner!'
Why on EARTH are you feeling bad about this? You KNOW that all the 'she's soooo much better' is a LIE- you know it for the best of all reasons- he wanted YOU, but you quite rightly decided he's not good enough. Pretending he's got what he wanted is his only face-saving option... but his cheeks are burning right now underneath as no matter what he says, he showed his hand to you- he knows you know he'd have come back if you'd let him.
So next time you see him, just grin. 'So glad my turning you down seems to have, errr, renewed your feelings for your lovely new partner. I think it's great that you can make the best of a bad job, eh? Good luck with it all, especially patching things up with the kids.'