If you used TurboTax CD/Download to prepare your son’s return, you would need to send him his .tt20 return file. By default, TurboTax will save return files to your Documents folder, inside a TurboTax folder, that itself has a folder for each year.
If you used TurboTax Online to prepare your son’s return, then he will have to prepare his 2021 tax return in your account if he wants to transfer in his 2020 information. There’s no way to move returns between accounts, so if he wants to use his own account this year, he will have to enter all his information manually.
You can still print or download a PDF copy of your son's previous TurboTax Online returns for his reference. Please see this TurboTax FAQ for instructions: In the online edition of TurboTax, how do I print a copy of my tax return?