You will not be able to e-file your tax return, it can only be printed and mailed claiming him as your dependent. You will need to amend his tax return so that it indicates that he can be claimed as a dependent.
See this TurboTax support FAQ for how to print and mail a tax return - https://ttlc.intuit.com/turbotax-support/en-us/help-article/print-file/switch-e-filing-file-mail/L5E...
