Welcome Megan! The NNAT is the Naglieri non-verbal ability test. Your DS getting a 99th percentile on that means if 100 randomly-selected children (his age or grade, depending how it's normed which I don't know) including him did the test, you'd expect him to score best or best-but-one.

If you Google

nnat site:davidsongifted.org

you get lots of threads about it, which you might like to browse.

More generally, hang around here, jump into any threads of interest, ask anything you like!

Email: my username, followed by 2, at google's mail