My son took the WJ-III Achievement test in December of 1st grade (age 6.5) and had similar scores to your DS. The report for DS states they used 'updated age-based norms' and his Grade Equivalent for Broad Math was in the mid 4th grade. My son had just finished EPGY's 3rd grade math curriculum, so IMO, the fact that your DS is scoring this high without receiving additional instruction in school clearly demonstrates how highly gifted he is in math. I sure hope the school provides differentiation in light of his scores.
The WJ-III scores have a mean of 100 and a standard deviation of 15, so the percentiles would be the same as for standard scores from an IQ test such as the WISC.
