If the 529 distribution was fully covered by his room and board, just don't enter the 1099-Q.
When the box 1 amount on form 1099-Q is fully covered by expenses, TurboTax will enter nothing about the 1099-Q on the actual tax forms. But, it will prepare a 1099-Q worksheet for your records.
On form 1099-Q, instructions to the recipient reads: "Nontaxable distributions from CESAs and QTPs are not required to be reported on your income tax return. You must determine the taxability of any distribution."
That said, if you still want to enter it, it's even easier, in TurboTax (TT), when the student is not your dependent. Enter the 1099-Q. When you tell TT that the student was someone you're not claiming ("someone else not listed here")*, it will give you a single screen to enter all the expenses, including Room & board.
*It may be necessary to delete your son from the personal info/dependent section.
