My understanding is that the NNAT is looking mostly at visual spatial or non-verbal reasoning so it would be reasonable to assume that your ds would do well on that part of an IQ test but doesn't give you a lot of info on his other areas of intelligence, but you probably already knew that -- lol!
Are you wanting to know how the NNAT might line up with the scores he'll get on the WJ? Was it the WJ cognitive (IQ) or achievement that he took? My kiddos have never taken the NNAT so I have no personal anecdotes to add to your guessing, but would venture to say that I understand that non-verbal/vs abilities tend to predict achievement less well than do other areas of ability so, if it was the WJ achievement, I'm not sure if it will tell you a ton about how he did on that one.
eta: Those are some pretty impressive numbers none the less

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