You did not really explain where your child was, and we could not tell from what you posted. The IRS does not care about custody agreements. The IRS cares where the child lived for the most nights. So again--if he was with you for at least 183 nights YOU are the custodial parent and you can claim your child on your tax return.
If the dad tries to claim him and happens to file first, then your e-file will be rejected. If that happens, print sign and mail your tax return so that it will be hand processed. Then the IRS will sort out who could claim the child. If the dad should not claim him he will face repayment of any child-related credits he received and perhaps some penalties too.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
