I think that he has a high P-MAP score. The students in grades 3-4, though probably took the other test version. They are not the same. Most of where he belongs really depends upon how students in HIS school score, as well as particular the demands of your DS's school curriculum.
How fluent is he in his addition, subtraction, multiplication, and division facts? How much writing and how much homework do the 3rd and 4th grade classes require? How comfortable is he with multi-step problems? These are just things I would consider.
