The amendment will have no affect on your ability to e-file a tax return. Since a tax return has already been received with his SSN, you cannot e-file a return with his SSN on your return.
You will have to print and mail your tax return, claiming your son as a dependent.
See this TurboTax support FAQ for how to print a tax return for mailing when using the online editions - https://ttlc.intuit.com/turbotax-support/en-us/help-article/printers-printing/print-mail-return-turb...
