Sorry no one got back to you. I'm not sure that you can tell whether a kiddo is gifted or not based on achievement scores, but I would definitely suggest seeking more of appropriate academics for yours. First discuss things with the teacher, and ask if they do any grouping for reading/math, etc. If they don't group in the classroom or if there's no other kids at his level, ask if there's any chance he could join the first (or maybe second?) graders for reading and math.
Along with Tamara Fisher, I agree that school should be about learning new things.
http://blogs.edweek.org/teachers/unwrapping_the_gifted/2012/09/what_it_really_boils_down_to.html Granted, kids learn a lot of other stuff in kindy (i'm assuming that's where your son is? I might have missed that), but if your kid already knows all the academic stuff they're teaching in his grade, the school should provide something else.
Was your testing done at the school or outside? Not all schools will look at outside testing, but the WJ is a common one in the school system, so maybe they will work with you if you share the scores. (The teacher won't care about scores, but maybe the school gifted coordinator/psychologist/principal will help you if you ask for advice on what to do with the scores, along with your son's complaints about the work he's doing and desire for more.)
ETA - where are my manners.... Welcome! Hopefully others will chime in with their experiences with the WJ and advocacy too.
