If these are SS based on age norms, it should not matter what grade he put in. If these age from grade norms, they may be a low estimate of SS. The KTEA and WIAT are similar in structure for math.
From a standard score/percentile standpoint, another test will probably not give you much more information, as few tests go beyond 160. Maybe the WJIII/IV. From an instructional placement point, I would suggest end-of-course placement testing, or readiness testing (e.g., pre-algebra), preferably tied to the school curriculum. Keeping in mind that problem solving encompasses the more critical reasoning and conceptual understanding aspect of math, I would look more to mps for a level. The num ops task is not really at the grade level that the GE suggests, since most children are not 100% accurate with skills at their grade level (and of course with all the usual GE AE caveats).
