Actually, that isn't how it's going to work at all. If he receives advance child tax credit payments, but ultimately isn't able to claim the child tax credit, he will end up having to pay all those back as part of his tax filing this spring. And you will get the full child tax credit. 

I have something similar but I am receiving the payments because I was able to claim my daughter in 2020. Would this mean if I opt out of the payments, that I wouldn’t receive any of the child tax credit at all or how does it work for alternating parents?