If you wanted to investigate advancing your younger DS's formal math placement, I would consider finding out if the school has placement tests for students who want to place into honors classes, and then asking if he can take them to place into the next level of math. E.g., if he exceeds the cut score typically used for placement into honors precalculus, then place him into precalc as a ninth-grader. They seem open to acceleration based on standardized test scores that they trust and understand. District placement tests would fall into the same category.
