In order to pay for your son's TurboTax fees he would need to have a refund to cover the amount of the fees & the refund processing fee which is $39.99.  You would choose that option when the program asks How Do You Want to Pay.  

Requirements for the Refund Processing Fee

• You'll need to e-file your federal return. It's not available if you paper-file your federal return.
• The Refund Processing Service option only appears if you have a federal tax refund, and you can only deduct your fees from your federal (not state) refund.
• You'll need to direct-deposit your refund into a single account, and the name on the bank account needs to match the name on the tax return.
• Refund Processing Service can only be used once to pay your fees.
• Fees (if any) are charged once, when the IRS processes your return. If your return is rejected, you might have to enter your bank account info again, but we won't charge you a second time.