I'm 18, and have been to WDW twice while a teen at ages 17 and 14 (i've been there 6 times total). If your nephew is like any "normal", non-addicted to WDW teen then he's going to do all of the thrill rides: Splash, Space, and Thunder Mountains, Alien Encounter, Test Track, Body Wars, Tower of Terror, Rock 'n' Roller Coaster, Star Tours, Dinosaur, and Kali River Rapids. These are all MUST DOs for teens. Another must do for teens is DisneyQuest; no teen can resist an unlimited amount of video games for one price (I know I can't) as well as taking him to the Water Parks. One thing you should NOT do is force him on rides that he finds "childish", i.e. almost everything in Fantasyland, that is unless he wants to go on them. Some rides I think he will enjoy regardless are: Pirates of the Caribbean, Haunted Mansion, Cranium Command, All of the Worlds in Epcot, Honey I Shrunk the Audience, Spaceship Earth, Backlot Tour, Muppetvision 4-D, Indy Stunt Show, Great Movie Ride, Kilimanjaro Safari, It's Tough to Be A Bug, and the Pangani Forest walk in Asia (I might be confusing the name of that, sorry if I did). Another good idea might be to take him to Pleasure Island one night, especially to the Comdey Warehouse and Adventurer's Club. I don't suggest those for kids under 10 or 11, so you might need to leave the young ones with a babysitter or one of the clubs at the resorts. Throughout the trip try to make sure that he's happy, but not killing your vacation; this is your family's vacation as well, just keep him pleased. As for eating, if he's into music-House of Blues, sports-All-Star Cafe or ESPN Zone, movies-Planet Hollywood, and in general: 50's Primetime Cafe, Whispering Canyon Cafe, and the buffet at the Beach Club Resort (forgot the name).
Again this is my opinion and taken from my experience, so I hope you got some good info from this.
