You should still claim your son since you have the right. The person who e-files first with your son will not get rejected, if he e-files first then you should still claim your son & you would have to mail in your return.
What if someone else already claimed my dependent
https://ttlc.intuit.com/questions/1899162-what-should-i-do-if-somebody-else-already-claimed-my-depen...
If you have the
right to claim a dependent claimed by someone else, then you must print and mail your return.
- The IRS will process your return and
pay your refund if you are entitled to one.
- Later, they will contact you and the
other taxpayer to determine who is entitled to claim the dependent. (The
IRS is not allowed to tell you the identity of the other filer.)
- The taxpayer who incorrectly claimed
the dependent will be assessed any additional taxes and penalties, and
required to file an amended return
-------------
Since you have the right to claim your son you can amend your last two returns to claim him. Once the IRS receives your amended return they will contact both of you to see who had the right to claim him. If they process your return your son's father would have to pay back any refund he received with interest & a penalty.
How to Amend (Change or Correct) a Return You Already Filed
https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed
