My son no longer qualifies as a dependent but he has already filed his taxes. What should we do?

If he already filed his return and indicated on it that someone else can claim him as a dependent, all he needs to do is to file an amended return to change that information. On your return, you would not claim him as a dependent. If you already filed your return you will need to wait to see if it was rejected or accepted. If they were rejected you are able to login to your return add the missing W-2, fix the errors that caused the rejection and refile the return. If your returns were accepted, then you will need to file an amended return.

You should not try to amend your return until the original return it has been fully processed and you have received your refund or your payment has cleared.Amended returns do not “catch up” to your original return and replace them; they are processed as two separate returns.  

Amended returns have to be printed and filed by mail.  It can take the IRS up to 12 weeks or longer to process them.   If you are due a larger refund than on your original return, your amended return should only show the difference and you will receive a separate check for it.  If you owe money on your amended return, it will show only the new amount owed, you will have to mail a check with the return. Click here for amending instructions: https://ttlc.intuit.com/replies/3288565