I am sure he will be bored in school during school year and wanted to propose to test him before each chapter and in case he shows he know it 100% give him play with AoPS Algebra and Alcumus.
My suggestion would be that you propose 80% be the cutoff for demonstrating mastery, and let the school negotiate up as high as 85% or 90%. Requiring 100% really feeds perfectionism, and if the school wouldn't require a kid with a B average repeat the course rather than moving on, they shouldn't require your kid to repeat the material, either.
I also wouldn't present it as "let him play," but rather "let him develop a deeper understanding of the material." I would not present it as "he will be bored," but as "he's already mastered the basics of this material, but we're hesitant to request a subject acceleration because we aren't sure what the options would be once he finishes geometry / we want to keep him with age peers / whatever."
My DD10 was supposed to have an in-class accommodation in lieu of a subject acceleration this past year, and it was not at all satisfactory, despite being fully supported by both the principal and classroom teacher. I think your plan of "let him work on this particular structured program" is more likely to succeed than the "here are some suggestions, but we're open to what the teacher comes up with" plan that we had. But there are a lot of places where any plan can break down, even with the best of intentions. "Everyone gets the same" is much easier than "one kid gets different."
