How can I claim my son when an ex partner claimed him first without my permission

If your son lives with you then you can claim him.   As far as the IRS is concerned the custodial parent is the one with whom the child spent the most nights--at least 183 nights in the tax year.   If someone filed first and your return was rejected then print sign and mail your return that claims your child.

Mailed returns are processed differently by the IRS.  It will take some time (maybe even up to a year) but eventually the IRS will contact both parties and sort out who could rightfully claim the dependent.  The person who wrongfully claimed the dependent will face repayment of refund $, plus penalties.

When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s.  Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.