Have your son been IQ tested? He seems to be pretty bright to be able to exhaust this content.
BASIS independent seems rigorous, however don't just jump to it, do you have any better options?
Perhaps you could ask them what accomodations they could give your child with ADHD. Considering their school regulations, they do remove disruptive students quickly. It seems as if they are not very tolerant. Is your son on medication?
I would probably find another type of school, or grade skipping to save time.
