I bet you a dollar he only has ADHD because someone thinks he should be doing something else with his time.
It angers me, that we live in a culture that believes it is perfectly acceptable to waste any amount of a child's time. Can you imagine being forced to sit still through grade 1 again, when you already know all of what is being taught? How do you think that's different for a child who has already learned everything they are teaching this year?
I have one brilliant friend (yes, I was tested in grade 4 to see if I should skip and the lowest score I got was grade 8 math because I didn't know what algebra was -- everything else was beyond 2nd year university) who describes kindergarten as that wonderful year when she got to learn to read. Again.
There is no room in the public school system for genius -- identified or not. They can't give your son the material that will challenge him, because then what would he do in grade 4 when he has to do it all again? They can't say 'hey, you're done, here's your grade 5 certificate, move on to middle school' because of the social realities of children in large, unsupervised, groups. They can't afford to give him a teacher to himself for 12 years, and even if they did, he'd be doing his master's program at, what, 12? His doctorate for his 16th birthday? The school is bound by the 'middle of the population' problem: most of the kids need something near the average, so that is literally all they can afford to offer -- both ends, in smaller and smaller proportions of the population, suffer but there is nothing that can effectively be done about it. When there are 2 kids out of 100 who can't learn and 2 out of a 100 who have already learned it, they need 5 teachers, and can still only afford to pay 3 -- even if 2 of them only have 2 students each. Besides, where is the cut off? The 'true' geniuses, the top 2% of the population, or the ones in the top 5? The top 10 could probably benefit from it, but what of the 11th? Someone's going to be ripped off, why not rip them all off evenly and offer, as it practiced, absolutely no special needs support for geniuses. Perfect. Since they're geniuses, they'll cope. And, as always, wasting a child's time is of no consequence -- it's not valuable time, it's not like they could use it more productively or more rewardingly doing something other than being bored out of their minds (or finding their own entertainment in the classroom as your son and my brother both did).
You work. You need your income, so you can't stay home with him, and can't afford to hire a tutor for your child to stay home and learn at his pace (which would be fast, and probably only take half an hour a day). If you could afford the kind of private school that could manage his brilliance, I'm fairly sure you'd have found that already.
So, he's stuck. You're stuck. The system is stuck. Yay.
Doesn't it make you angry?
I am still angry about the waste of MY time school was. My kids didn't go to school. I read the curriculum from time to time (I actually read the whole thing all the way through, once -- man, that's hilarious! Apparently, it was written by a committee of people who have only ever heard the sounds 'correct grammar' and 'proper English' but have no idea what those sounds mean!)... in reading the curriculum I found that my kids were AT LEAST two years ahead all the time, and we used no curriculum or structure of any kind at home -- pure interest-led learning based on what they wanted to do with their time, all day, every day. (Apart from regular family-life stuff, chores, grocery shopping, things no one thinks of as 'educational.')
The last hilarious thing that happened was when my now-17yo was 14, helping a good friend of hers with her grade 12 homework. It was Spanish, a language both of my kids pretty much only know exists from tv like Dora. And math. My 14yo 'never interested in math in any way, doesn't do any that doesn't involve money' was helping a 19yo with her grade 12 advanced math homework. Ah, the irony!