I should underline that I am not remotely downplaying the OP's child's mathematical gifts. Merely affirming that accurately determining instructional placement will require a different kind of assessment, preferably one based on the curriculum to be used. If you have access to placement tests for the curriculum used by the school, that would be the most straightforward approach to finding his level. Alternatively, if the school were amenable, he could compact through the curriculum by taking chapter/unit tests in sequence until his accuracy drops below 70%, which would also be an approach to identifying his instructional level.
