Magic Kingdom Security is in charge of this ceremony each day.
If there isn't already someone signed up for that day, he may be able to participate.
Our Boy Scout Troop did the flag retirement twice on our trips to WDW. Once in June 2001, and again in June 2007. It was lovely both times. Very memorable.
Ask on your first day in the parks. Even if someone is already signed up, they may be able to put him in on another day of your vacation.