Child did not check someone else can claim him as a dependent

Sorry---you have just joined the large club of parents whose ability to e-file has been blocked by their child's mistake.

The ONLY way you can file this year claiming your son is to file by mail.  No amount of waiting for him to amend will help you e-file, even later.   He has put his SSN into the system.  Any attempts to e-file another tax return using his SSN will be rejected this year.  Print, sign and mail your return that claims him. 

He will have to wait for the IRS to fully process his return, then amend.  He has to go to My Info, click his name and change the way he answered the question that asked if someone else can claim him.  He has to say YES.

See this TurboTax support FAQ for amending a tax return - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-change-correct-return-a...

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 UPS or 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.