it would seem there's a tie in who can claim the child for 2020 since both you and his mother meet all the tests for claiming the child
in such a case the IRS has tie-breaking rules
and the rule that comes into play is which of you had the highest adjusted gross income for 2020.
since you say she didn't work, it would seem you get to claim the child.
you'll need the child's social security number and you'll have to paper file.
eventually both you and her father will get an IRS letter asking to amend - to not claim the child
if neither amends the IRS will send a second letter to both asking for proof. the letter will specify what proof to submit.
