I filed my sons return......how do I file another return for son

If you use TurboTax Online, you can prepare one federal e-filed tax return and up to five associated state tax returns per account. 

If you choose to file another federal return using the same TurboTax Online account, it will clear your information and you’ll lose access to your prior year's return(s) for future use.

Instead, create a new TurboTax Online account for each federal tax return prepared.  You’ll be able to use the same email address if you wish, but a unique user id and password is created for each new account. The links provided below provide more detail on TurboTax Online accounts and how to create new ones. https://How many times can I e-file using TurboTax Online?

https://How do I start another return in TurboTax Online?