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-a...
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-a...
