If you are eligible to claim him, do so. But you will have to file by mail since his SSN has already been filed. The IRS will process your return. Then later, they will both of you letters to request one of you amend your return.
The parent that the child spent the most night of 2020 with is eligible to claim the child. The non-custodial parent will need to pay back any credits they do not qualify for. @Sfeffggg
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