If he's doing a computer adaptive program, can he do it at a higher grade level? Another option is Khan Academy...just make sure you put him at the correct grade, not 4th grade.
The problem, as you might also experience, isn't the starting level so much as the amount of repetition. The teacher seems to think that when they use Front Row (which is adaptive and pre-tests them into a level where they need work) it serves as enough differentiation -- but it's asking him to do tens, if not hundreds, of problems on topics that he grasped within the first few minutes of instruction, you know? And they do Khan sometimes, but you're exactly right that subject acceleration is no good if the pace is still too slow (and the instruction is still overly procedural, which I find is the drawback of Khan.)
I'm kind of liking the flipped classroom suggestion. He loves Alcumus...if we were permitted to forego homework, I could possibly teach him AOPS Prealgebra and then let him do Alcumus during class, especially when they have Chromebook time anyway. Hrm.
I'm super jealous of your kiddo's individual instruction.
Good luck with next year!
