Greetings. First, I would tell you that this issue should have been dealt with in your separation agreement, so you may want to contact your attorney to see if that was taken care of.
If it was not taken care of, the rule is that you must have her consent to claim her and her son OR you must file jointly. I would suggest that you file jointly, especially if you will have to pay. This may obligate her to pay for half of the taxes due though, so you may want to amend your separation agreement to deal with this issue. Best of luck!
Janet L. Fritts
Attorney at Law
4101 Lake Boone Trail, Suite 200
Raleigh, North Carolina 27607
The response posted above is based upon the limited factual information made available and is not intended as a full and complete response to the question. The only reliable manner to obtain complete and adequate legal advice is to consult with an attorney, fully explain your situation, and allow the attorney sufficient opportunity to research the applicable law and facts required to render an accurate opinion. The basic information provided above is intended as a public service but a full discussion with an attorney should be undertaken before taking any action.