To get a rough idea of what he has mastered, you need to find out what grade level and time of year his RIT score is at the 90th percentile (students at the 50th percentile haven't mastered grade level material). Using the 2011 norms, this would be somewhere in the fall of 3rd grade. Since the 2015 norms would probably place him slightly lower, he is likely ready for 3rd grade math.