If you are going to file as Married Filing Separately you will need two tax returns.
Using the TurboTax online editions you can only complete one tax return per account and User ID. If you want to start another tax return with the online editions you will need to create a new account with a new User ID.
Go to this website and click on Sign up in the upper right of the screen and create an new account - https://turbotax.intuit.com/
Please read this TurboTax Help topic on how to start a new tax return for another person.
If you are using TurboTax Online, you cannot prepare and file a tax return for another person using an existing TurboTax account. You'll need to create a new account for yourself, distinct from the account you used to file your son's return last year.
If you are using TurboTax Desktop, you can start a new return for yourself by clicking on Start a new return on the Welcome page of TurboTax.