He has to file an amended return changing his dependency.
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-already-filed/L4VjJ9BA2_US_en_US?uid=ld1n99kr
You enter him as your dependent on your tax return. You cannot e-file your tax return because the IRS has already received a tax return with his Social Security number and even when he files an amended return that will not change the IRS database for received tax returns.
You will have to print, sign, date and mail your tax return to the IRS.
See this TurboTax support FAQ for how to print and mail a tax return using the online editions - 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-already-filed/L4VjJ9BA2_US_en_US?uid=ld1n99kr
Did your return reject because he claimed himself? Sorry, you now have to print and mail your return. You don't have to wait for them to amend. Even if they amend, it won't release their ssn in the IRS system.
How to switch from Efile to mail. This one has a Video
How to mail a return from the Online version
Be sure to attach copies of your W2s and any 1099s that have withholding on them. You have to mail federal and state in separate envelopes because they go to different places. Get a tracking number from the post office when you mail them for proof of filing.
Son - How to Amend this year