My DD took explore in 3rd grade. It was very good for her. Just be sure to stress that this test is for fun, and there will be lots of things that he doesn't know how to do.
It was the first time she took a test that was difficult for her. She didn't really like the way that felt, but it did help her have some empathy for her classmates during test situations.
When you register they send you a few sample questions. I wouldn't recommend "studying" for the test, beyond what they send you. After all, you want to know what you child can do right now, rather than what they can do with test prep.
NUMATS will send you an educational plan along with the scores. We found that to be useful in helping to advocate. An IQ score is one thing, showing specifically what you are ready to learn is even more convincing.
We haven't done any of the online courses. They seemed too expensive, and I'm not sure my dd would enjoy them. We've chosen to enrich with camps, art classes, etc., that are taught in person. I guess that just depends on whether you think your child would enjoy an online experience.
