After submitting this question, I found the answer in IRS Publication 17 (Form 1040 instructions page 12):
Third Party Designee
You can authorize the IRS to discuss your return with your preparer, a friend, family member, or any other person you choose (other than your paid preparer) by checking the "Yes" box in the "Third Party Designee" section of Schedule 6. Also, enter the designee's name, phone number, and any five digits the designee chooses as his or her personal identification number (PIN).
If you want your paid preparer to be your third party designee, check the "3rd Party Designee" box on page 1 of Form 1040. Do not complete Schedule 6.
So, since I did not complete my son's form as a paid preparer, Turbo Tax was correct in leaving the 3rd Party Designee box unchecked on page 1 of Form 1040. But I'm assuming that it will include Schedule 6 with my designee information when I e-file his return. It would be helpful if TurboTax did include these designee items as interview screen questions (instead of worksheet-only form entries) in a future year's update.
