If you purchased a desktop version of TurboTax (via a Download or a CD), you can use it to prepare and file up to 5 federal returns, so you'd be able to file your son's taxes using the same copy of TurboTax that you used.
TurboTax Online is a little different though, in that you can only prepare one federal tax return per account, so if you used TurboTax Online, you'll need to create a new account for your Son's tax return.
