If was under the age of 19 for tax year 2021 then he can be claimed as a dependent under the Qualifying Child rules. If he did not provide over one-half of his own support and lived in your home for over 6 months of the year, among the other rules, then you can claim him as your dependent.
To be a Qualifying Child -
1. The child must be your son, daughter, stepchild, foster child, brother, sister, half brother, half sister, stepbrother, stepsister, or a descendant of any of them.
2. The child must be (a) under age 19 at the end of the year, (b) under age 24 at the end of the year and a full-time student or (c) any age and permanently and totally disabled.
3. The child must have lived with you for more than half of the year. Temporary absences while away at college are considered living with you.
4. The child must not have provided more than half of his or her own support for the year.
5. If the child meets the rules to be a qualifying child of more than one person, you must be the person entitled to claim the child as a qualifying child.
6. The child must be a U.S. citizen or U.S., Canada or Mexico resident for some portion of the year.
7. The child must be younger than you unless disabled.
If he meets the requirements to be claimed as a dependent Child, then he will have to amend his 2021 tax return to change his dependency status, assuming he filed as Not a dependent.
If his tax return has been accepted by the IRS then you can only print and mail your 2021 tax return since the IRS has already received a tax return with his Social Security number and not a dependent.
See this TurboTax support FAQ for amending a 2021 tax return for a return originally completed and filed using TurboTax - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-2021-return/L1gEU4G3X_U...
See this TurboTax support FAQ for how to print and mail a tax return using the online editions - https://ttlc.intuit.com/turbotax-support/en-us/help-article/printers-printing/print-mail-return-turb...
