MICHAEL JACKSON'S oldest son, 16-year-old PRINCE MICHAEL, has lined up a gig as a "special correspondent" for "Entertainment Tonight". Since he's still pretty young, he probably won't appear on the show regularly . . . but it sounds like this IS an open-ended gig, not just a brief stunt. Prince Michael made his debut on last night's episode. His first task is to cover the upcoming movie, "Oz the Great and Powerful", which has some significance to him since his dad played the Scarecrow in "The Wiz".