Additional filing for more than 1 person on the use for free program of turbotax 2019

No, you cannot file using your son's account.  

Each tax return requires a separate account with a separate username and password, but you can use the same email address for all of them. Never start another return by going back and overwriting everything in the return you just filed. This will replace your info, which means you won't be able to retrieve your return later.

You might want to switch to the TurboTax CD/Download software to save money

• Unless the additional return/s you want to file in TurboTax Online are simple returns and can be filed using the Free Edition, there is a charge for each additional return filed in TurboTax Online.
• On the other hand, the TurboTax CD/Download software lets you file up to 5 federal returns at no additional cost.

If you still want to start another return in TurboTax Online, here’s what to do:

1. Sign out of TurboTax Online.
2. Select the TurboTax Online product you want to use for the second return and proceed.
3. On the Create your account screen, set up a new user ID for the second return.
   • You can reuse your email address if you want to receive e-file notifications for the second return.
   • Up to 5 accounts can be tied to the same email address.
4. After you've created the new account, you can start working on the second return.
5. If you can't create a new account because your web browser automatically logs you into your old account, you may need to clear your browser history, cache and cookies.