Our DS8 (when 7) got a higher score in Broad Math. He's done grade 7 math (while officially a 2nd grader) and found it pretty easy, though the course only had routine questions. So your DS could probably handle multi year acceleration (i.e. not just one or two years, but a bunch) assuming you can quickly fill in gaps.

See also this thread
http://giftedissues.davidsongifted.org/BB/ubbthreads.php/topics/183501

BTW I think the WJ-III achievement test for math must have some norming problem and gives out scores that are higher than they should be, but regardless, multi year acceleration is warranted if it can be done properly.