Hi Jullie:
There are plenty of teen-friendly, fun, and non-character (I'm guessing your son acts like he's not into the characters as much as he was when he was a toddler, right?) restaurants throughout Disney World.
If you have a sophisticated soon-to-be 14 year-old, check out California Grill atop the Contemporary Resort. California Grill features delicious California Fusion cooking and spectacular views of the Magic Kingdom and the Wishes fireworks show. I would also recommend Le Cellier, a tasty steakhouse at Epcot's Canada Pavilion, Jiko at Animal Kingdom Lodge, which serves dishes influenced by African, Indian, and Mediterranean cuisine, 'Ohana (for dinner), a Polynesian family-style restaurant, and Coral Reef, a wonderful and fun experience where you can dine in front of stunning underwater views.
More suggestions on the fun and slightly-less-sophisticated side are: 50's Prime Time Café, American comfort food and fun service in a period setting; Whispering Canyon Café, an Old West themed family-style restaurant; Raglan Road, a modern twist on traditional Irish pub food; and T-REX Café, a dinosaur-themed family restaurant with a large menu.
For this special birthday, I would also consider the
Hoop-Dee-Doo Musical Revue Dinner show. Hoop-Dee-Doo is a vaudeville-style dinner show with a country twist. Dinner is served family style and includes favorites like fried chicken, smoked bbq pork ribs, fresh corn bread, strawberry shortcake and the most delicious country-style baked beans I have ever tasted.
Enjoy the special day.