I did not claim my baby born in 2020 as a dependent, his father did. His father taxes got rejected but mine didnt. How do I amend my taxes with the 1040x form?

I entered in my taxes I had a baby in 2020, and his ss #. I selected no to not claim him as a dependent.  Babys father did claim him as a dependent, but his taxes were rejected because babys ss# was already on my taxes. How do we amend this issue so babys father can refile?