How to show IRS I used 529 plan distribution to pay son's student loans?

As explained at your other post*, you can't tell the IRS that, unless you want to mail in your return and attach a statement.  When the box 1 amount on form 1099-Q is fully covered by expenses, including student loan payments, TurboTax will enter nothing about the 1099-Q on the actual tax forms. . But, it will prepare a student info (or 1099-Q) worksheet for your records. The worksheet is not sent to the IRS when your e-file. 

If you want to get that work sheet, enter the 1099-Q at Deductions and Credits / Education / ESA and 529 Qualified Tuition Programs (1099-Q). Follow the interview which will allow you to enter the loan payment. Do not enter anything, later, at the educational expenses section. Be forewarned, it's easy to make mistakes in that long complicated interview. 

* https://ttlc.intuit.com/community/college-education/discussion/re-i-have-funds-in-my-son-s-529-plan-i-want-withdraw-those-funds-and-pay-my-son-s-student-stafford/01/2718434#M48020