So you are the custodial parent. That is all the IRS cares about. If your ex tries to e-file, ex's return will be rejected--same as yours will be. When your son "claimed himself" he put his SSN into the system; anyone else who tries to claim him as a dependent will be rejected. Your son should not have claimed himself. Only about $500 of his wages were even taxable, so he needs to amend.
But in order for you to claim your son--as you should be able to do--you have to file on paper. You still can claim him and get the $4050 personal exemption for him to lower the amount of income your are taxed on, the child tax credit and possibly earned income credit if you qualify. So print sign and mail your own return claiming your son. You do not have to wait for your son to amend; you can go ahead and mail in your return.
As for what your ex does, if he files a return claiming your son, (and figures out he had to mail it) then eventually the IRS will send you both letters to sort out who could rightfully claim your son. Since it is YOU, ex will face possible penalties and repayment of some of his refund for claiming a dependent when he should not have.
**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.**
