claiming a qualifying dependent

Is your grandson under age 19 or is he a full-time student under the age of 24?   If he can be claimed as a dependent and made a mistake on his own tax return, he will need to amend and change the way he answered the question in MY INFO about whether he can be claimed as someone else's dependent.

But---since he already used his SSN on a tax return, you cannot e-file your return that claims him.  The only way you can file and claim him as a dependent is to file your own return by mail.  Waiting for him to amend will not help you.   Print, sign and date your return in ink, and mail it in.

When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s.  Use a mailing service that will track it, such as certified mail so you will know the IRS/state received the return.

Federal and state returns must be in separate envelopes and they are mailed to different addresses.  Read the mailing instructions that print with your tax return carefully so you mail them to the right addresses.

He has already filed his taxes.  If you haven't already filed, follow the steps below.

Confirm that he dose fulfill the criteria for a dependent based on the rules stated at the bottom.

• You will amend his return and make the "someone else can claim me as a dependent" question.
• How do I amend my federal tax return for a prior year?

• You will have to mail your tax return.
• TurboTax will give you directions when you print it out
• E-File will fail because his SSN is already in the system.

You can claim your Grandson as a dependent provided he meets the rules below.

• Your son must be a U.S. citizen, a U.S. national, U.S. resident, or a resident of Canada or Mexico.  There are certain exceptions for adopted children.
• You can't claim someone who is claimed as a dependent on another tax return. 
• You can't claim someone who's married and files a joint tax return. 
• They are related to you? The child can be your son, daughter, stepchild, eligible foster child, brother, sister, half-brother, half-sister, stepbrother, stepsister, adopted child or an offspring of any of them.
• Your grandchild must be under age 19 or, if a full-time student, under age 24. There's no age limit if your child is permanently and totally disabled.
• The grandchild must live with you for more than half the year. Living at school as a full time student does qualify.
• They may have a job, but they cannot provide more than half of their own support.

See the TurboTax article Rules for Claiming Dependents on Taxes