If their answer truly is that they will do nothing to support his needs, then I'd enroll him in a Beast/AoPS course, explain the situation to him, and ask him to just put up with it.
Fighting the school, telling them he's not going to participate, etc., likely does not lead to a good outcome - short or long term.
This is exactly what we did 5 years ago (pre Beast Academy).
Even acceleration using regular school maths would not have helped because that curriculum is more of an industrial ‘production line’ approach which appears designed to (or is certainly taught this way) to cater to an averagely able and motivated student.
Doesn’t sound as though such an approach would be optimal for your child either. For our DD it was tough at first because girls appear to be under far more pressure to conform and dumb down. She adjusted quickly though once we got underway.
