Debbie,
My son�s scores were variable on the Terra-Nova when he was in the early grades. For example, in 2nd grade, his science score was well below average. We didn�t know what to make of those results other than we believed the must be inaccurate because he had always had an advanced understanding of science. At some later date, I flipped the results over and read the descriptions of the general content and administration. For the 2nd grade science test, the teacher read an (excruciating simple, I�d imagine) question aloud and the students had to chose the best pictorial depiction of the correct answer. He probably would have done very well on the fourth or fifth grade version. They were just asking ridiculous questions.
I agree with Dottie, go two years higher if you can. Your son will have more room to show what he knows and he will have higher interest in the task!