you can't stop her from claiming your son. if she has e-filed and you try e-filing your return claiming your son it will be rejected. (the first to file gets to e-file. the second to file must mail in their return.
eventually, the IRS will send each of you a notice asking you to amend. if neither amends, then the IRS sends a second notice asking for proof you're entitled to claim the exemption. the one with the best proof gets the exemption the other gets a bill.
here are the rules for claiming a child
1) be the taxpayer's child, stepchild, eligible foster child or descendant of any of them such as a grandchild or brother, sister half-brother, half-sister, stepbrother, stepsister or descendant of any of them such as a niece or nephew, legally adopted child or place with the taxpayer for legal adoption
2) be any of the following a) under age 19 at the end of the year b) a full-time student and under age 24 at end of the year c) or any age if totally and permanently disabled
3) child must have the same principal abode as the taxpayer for more than ½ the tax year.
4) the child hasn't provided over ½ his/her support
5) No joint return with the exception for a joint return filed only to claim a refund of tax withheld when neither spouse is required to file and no tax liability would exist for either spouse if separate returns were filed
6) Not be a qualifying child of another taxpayer with a higher priority under the tie-breaker rules
if these tests are met he's your qualifying child. your mother can't claim your son because he's your qualifying child. I won't go into whether it was proper for your mother to claim that son in prior years. however, if she doesn't qualify and is caught, she could face substantial penalties. she would lose any child credits claimed and her status would probably go from head of household to single resulting in higher taxes.
