Eh, I wouldn't worry about it. There is so much stuff at Disney World that is fun and doesn't necessarily center on the characters, the movies/storylines, etc. Obviously what he will be able to go on will depend on his size, but my stepson was able to go on most stuff when he was 6 and he was on the small side.
Magic Kingdom - Space Mountain, Big Thunder Mountain Railroad, Splash Mountain (which does use characters from Song of the South but so few kids these days have seen the movie I doubt it would matter), Pirates of the Caribbean, Haunted Mansion...plus some of the smaller attractions like Astro Orbiter and Tomorrowland Speedway. Buzz Lightyear Space Ranger Spin is really fun, even if you are not a Buzz Lightyear fan.
Animal Kingdom - Dinosaur, Expedition Everest, Kiliminjaro Safaris, Kali River Rapids
Hollywood Studios - Tower of Terror, Rockin' Roller Coaster, Star Tours, Indiana Jones, the stunt show...also Toy Story Mania, even though it has characters from Toy Story, is a blast!
EPCOT - GM Test Track, Mission Space, Soarin'
There are the Universal parks as well, with Harry Potter and all the rest.
I think sometimes Disney is just one of those things that's hard for kids at that age to totally wrap their heads around. If he's not crazy about the movies, he may think that's what its all about, and of course that's not true. You can try going on the Disney website and showing him the different attractions, I think there is a DVD you can send away for as well. Or you could just not say anything until you actually get there. I certainly would not worry about him having a bad attitude in front of your family - I mean, you can't control how he reacts to things and Disney can be overwhelming and tiring. Be sure to take a break in the middle of the day to head back for some rest and swimming in the pool.
For what it's worth, a friend of mine once planned a family trip to Jamaica. When she first asked her son if he wanted to go on a trip to Jamaica, his reaction was NO - because he really had no idea what it all meant. So she was all worried about him, because they were going anyway, but I just told her, look, he really has no idea what he is talking about, because he couldn't possibly realize how awesome it would be to take a trip there. He was 9 years old and had absolutely no perspective, like a teenager or adult would. They ended up still going and he had a great time!
ETA: "Unofficial Guide to Disney World" is a must! There is also an "Unofficial Guide to Disney World With Kids" too!