My son (16), filed his taxes but didn't choose that he can be claimed by someone else. His taxes have already been submitted. How can we fix it so that we can file?

[rjs]

There is nothing you can do to be able to e-file your tax return. After your son's tax return has been processed and he has received his refund he will have to file an amended return to show that he can be claimed as a dependent. But that will not let you e-file your tax return. You have to file your tax return by mail.

[LaShaunA3]

You can still claim him, but if your e-file is rejected due to his social security number already being used, you’ll need to print and mail your return. 

He should file an amended return (Form 1040-X) to indicate he can be claimed as a dependent on another return.