Do you have any testing from more recently? It's possible that the testing he did a 4yrs old doesn't accurately reflect his abilities.
Second, it's possible that your DS is just trying to entertain himself. It's hard for anyone, much less a 6yo boy, to sit 6 hours a day five days a week listening to information that you already know.
ADD-inattentive could also be an issue, but I'm not sure if you can get an accurate read on that until you see him in an appropriate learning environment.
How is his behavior at home?
