Assuming that the child lives with you and you meet all of the requirement to claim the child as a dependent.
Then unless you have a pre-2009 divorce decree that specifically states that he can claim the child with no conditions attached, or you have previously given him a signed 8332 form releasing the child's exemption for all future years and you have not revoked that in writing, then unless you give him a current 8332 for releasing the child's exemption (dependent) to him, he cannot legally claim the child since he must attach that 8332 form to his tax return to claim if he did not physically live with the child more than 1/2 the year.
What you can do is:
1) e-file before he does and claim the child.
2) if he e-files first then your e-file will reject because your son's SSN has already been used.
3) If that happens, then print and mail your return claiming your son.
4) The IRS will pay both refunds, but within a year (up to two years possibly) both parents will receive a letter asking if the child was claimed in error. After you answer that letter that the child was not claimed in error then if the other taxpayer does not amend, the IRS will follow up with a 2nd letter asking for more details. Simply provide the information that they ask for to supports your claim that the child lived with you and you did not release the child's exemption. (Don't ignore the letters or you will loose).
5) The IRS will determine which tax payer gets to keep their refund and which must pay it back along with interest and possible penalties. The parent that the child lived with will almost always win.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
