I filed my taxes but I didn't file the dependent part right and need to fix it

If you've already filled the return, you will have to do an amendment.

The April 15th deadline dose not apply

Wait for the original return to be completely processed.

TurboTax Online

1. On the Tax Home screen, scroll down to Your tax returns & documents and select the year you want to amend.
2. Select the Amend (change) return dropdown, 
3. Then Amend using TurboTax Online. 
4. If you're told to amend your return via TurboTax Desktop, follow the steps for TurboTax Desktop instead.
5. Continue through the screens,
6. Make the changes you need to make, and carefully answer the remaining questions to finish amending your return.

Go back into his tax return.

1. Under My Info (Top of left sidebar)
2. Click the down arrow next to his name
3. Scroll down to add a dependent