If you intend to use the WIAT as a progress monitoring tool (which, of course, is not its designed purpose!), then you want the report to include growth scores, as well as the usual standard scores, so you can demonstrate that his absolute skill level has or has not moved forward.
When advocating for advancement, I think schools are more readily convinced by testing from within their own curriculum, such as unit assessments, end of course, or placement exams. It's also easier to identify a destination instructional level.
