I presume you are stating you have "primary custody" because the divorce decree awards you the right of custodial parent, or under IRS rules the child resides with you more than 1/2 of the year or at least more days per year than with him. If so, then he has no right to claim the child on his tax return. Also, he has no right to forge your signature on his tax return. What can you do? - Practically, the only thing you can really do is file your own return and claim the child on your return and let the IRS sort it out. They will send him a notice, and you too, once the IRS computer sees the contradiction in the returns, and you can show the IRS your proof, and he won't have any to show his position is correct, and he will be assessed additional tax and penalty for the false claims.
This information is provided for educational purposes only, and is not to be relied upon as legal advice. You should consult with an attorney with full disclosure of all facts and opportunity to consider all or alternative options.
Your question is a little unclear because you use the term "separated" but also "ex husband" - in Texas you are either married or not married - there is no legal status of separated. Is he now your ex-husband, but filed a return for a time before you were living apart, but not yet divorced? Was he your ex-husband at all times referenced in your question? Or, do you not yet have a final decree of divorce, but you refer to him as your ex-husband because you no longer live together?
If he filed as married, when you were actually divorced in that tax year, you need to get your own tax return on file for that year as well as this year. If you were married during that tax year, half of any tax refund would have been your community property - if you are officially divorced now, you should check your decree to see what it says about tax refunds. If you are not officially divorced with a final decree, you need to consult a family law attorney about getting that done and address all of these tax issues with him or her.
This answer is made available for educational purposes and is not intended to give you specific legal advice. I do not represent you and by using or participating in this site you understand that there is no attorney client relationship or privilege between you and me. This site should not be used as a substitute for legal advice from a retained, licensed attorney in your state who has reviewed your question and the surrounding facts in detail. I attempt to provide good, helpful information, but do NOT rely on anything I write - you should use my comments as a starting point for consulting an attorney you have retained to represent you regarding your specific legal and factual circumstances.
If you're still married and not yet legally separated or divorced, then you must file as Married, either Jointly or Separately. And you must both agree to file Jointly if that's the choice you make. Otherwise, you'll need to file as Married, filing separately. If the divorce is final then you can file single or as head-of-household. Who claims the dependent exemption for the child depends on the divorce decree if there is one and the IRS rules. Check www.IRS.gov.
Evan A. Nielsen is licensed to practice law in California and handles federal tax matters throughout the U.S. The information provided here is for educational purposes only and is not intended as legal advice for a particular matter. This response does not create any attorney-client relationship with the author. For specific advice about your particular situation, please consult an attorney.
Sign up to receive a 10-part series of useful information and legal advice about the divorce process.