My kids took that test last year. I think I can help you interpret the scores.
The results don't mean that your son is performing AT a fifth grade level or a 13th grade level. What they mean is that a 5th grade student who was four months into the school year would be expected to get the same score that your son did.
My eldest scored at a mid-12th grade level on the math portion of one of those tests last year. He was barely 9, and he definitely wasn't doing calculus yet! He just got all the questions right, and the people who created the test had decided or found that his score would typically be expected from a 12th grader.
HTH.
Val
