Oh dear. You have my sympathies, you could be describing my DS1, right down to the fact we were told by both his pre-school and his primary school (at various times, but several teachers), to stop answering his questions, to not let him read grown-up books and magazines and to not watch documentaries on tv!
He was bullied at and hit by one boy (1st year of high school) every time he scored well in a class test, this boy actually tried to make him write wrong answers.
DS1 Tested 8 years above his age at age 5 1/2, and not surprisingly hated school, though everyone was "stupid" (not nice for him to say, but that was the frustration talking). Work was supposed to be differentiated at school after the testing (done at the request of the school), but the teacher refused to do this on the grounds that DS1 refused to do the easy work first (bored to actual tears). Had an awful couple of years in primary, then we changed schools as he had gone into full school refusal mode, being sick every morning etc. New school was a lot more supportive but not perfect - still a lot of resistance to teaching him to his potential. By mid way through high school, he was pretty much self taught, allowed to do his own research in school when school tasks were done, and he was a much happier boy (the teachers realised he needed to DO stuff and a lot of that stuff was beyond them, all but 1 teacher in his last year were happy to let him read and learn ahead). He did a number of extra subjects to exam level, at school but without taking the classes, all self motivated learning.
DS1 is now 18 and a Student at Oxford. It has been a long and very tough journey for him, and us, but he is so HAPPY surrounded by lots of people who are just like him for the first time.
Obviously, the social side of things, you need to handle very carefully, your DC may be very clever, but he is still a 6 year old. He needs to taught to respect other kids and their abilities. Tell him some kids learn faster than others, some need more practice, some need less, but that doesn't mean he is better than them, and stupid is not acceptable to say to people. Computer games and sports are things that he can have in common with other DC of different abilities, make sure he knows how to do "small talk" and draw out the other childs interest. He may find it easier socially to chat about football/computer games with other DC, this is not the same as "dumbing down", he should be encouraged to be proud of his abilities and to work hard,
and always do his best, but he needs to learn how to interact with other DC is own age - and I do appreciate that can be very difficult when there is a real gulf in where he is academically compared to most of his age-peers.
The history channel (and similar), QI (and similar), The New Scientist (and similar), are all fantastic resources, if he's not ready for them, they won't hold his interest. My DS loved all that sort of thing from age 5/6, would have went barmy if (as the nursery/school suggested), we didn't let him read/watch. He needed that mental stimulation.
BTW, I also have 4 v. bright DS!
Good luck!