If your nephew met the requirements to be claimed by you, he should have indicated on his tax return that he could be claimed by someone else. If he did meet the requirements but filed incorrectly, he will have to prepare, print and mail an amended return form 1040X to check the box that he can be claimed by someone else.
Since his mailed amended return won't be processed for many months, you will have to print and mail your return to get it accepted if you claim him as a dependent on your tax return.
Here are the requirements for you to claim a full time student under the age of 24:
1. The child must be your son, daughter, stepchild, foster chld, brother, sister, half brother, half sister, stepbrother, stepsister, or a descendant of any of them.
2. The child must be (a) under age 19 at the end of the year, (b) under age 24 at the end of the year and a full-time student, or (c) any age and permanently and totally disabled.
3. The child must have lived with you for more than half of the year. Temporary absences while away at college are considered living with you.
4. The child must not have provided more than half of his or her own support for the year.
5. If the child meets the rules to be a qualifying child of more than one person, you must be the person entitled to claim the child as a qualifying child.
6. The child must be a U.S. citizen or U.S., Canada or Mexico resident for some portion of the year.
7. The child must be younger than you (or your spouse, if filing jointly) unless disabled.
