I think you need to figure out what it was that he didn't like at 6. Not saying he will love all rides, but by determining what it is he didn't like, you may find some rides he may like that are different from what he did before. Also, something he might have been scared of or bored by at 6, he might actually like at 12. There's a big difference between a 6 year old and a 12 year old. I'd point that out.
Maybe he'd make an exception for Star Wars, POTC, TT, or some of the coasters.
Has he ever been interested in space or being an astronaut? The Mission Space ride where it simulates going to the moon might be fun for him. It isn't a typical kiddy ride, and if you do the green side, it isn't an intense coaster type ride either.
As far as non rides:
There are shows. Lion King and Indiana Jones would be good for a 12 year old boy, not too babyish, girly, or boring. The Monsters Inc comedy show is fun at MK. Also, maybe check out the smaller shows and demonstrations at World Showcase. We always enjoy the Chinese acrobats.
Try filling time with parades and fireworks, or maybe plan some long sit down meals. Boys usually like things that involve food. You could try one of the dessert fireworks parties.
Try out some of the interactive / scavenger hunt type games like Sorcerers of the MK, Pirates Adventure, or Agent Ps Showcase.
Visit one of the water parks or plan time at the resort pool.
Try the virtual reality experience at DS, the Void.
Visit FW, hiking, fishing, horseback riding.
Rent boats or bikes.
AK is a glorified zoo. Does he enjoy animals? There's a bird show, animal trails, and a safari ride.
There's a small aquarium at Epcot if he likes fish.