Sorry, but no. Only 1 Federal return can be prepared per Online account. If you try to prepare a second one in the same account, it will overwrite the first one. You can set up a new account to prepare a return for your son. Depending on the complexity of his return, he may be able to use the Free Edition.
You can use the same email address for notification purposes in up to 5 accounts, but each User ID has to be unique.
FAQ: How do I start another return in Online TurboTax?
https://ttlc.intuit.com/community/using-turbotax/help/how-do-i-start-another-return-in-turbotax-onli...
NOTE: Also see the information in my next comment that follows below if your son's only income is unearned income.
This answer has been verified for accuracy by an Intuit expert employee
