I'm filing my son's PA state tax return. I've already filed his federal return with another online filing program. How do I file ONLY the state return with Turbotax?..

 When you use TurboTax the information flows from the federal return to the state return.  You cannot use TT to prepare a "stand alone" state return.  Try going to your state's tax website to see if you can file there.

CONTACT THE STATE

https://ttlc.intuit.com/turbotax-support/en-us/help-article/state-taxes/contact-state-department-revenue/L9qVToi02_US_en_US?uid=m6e06um0