Hmm....I don't think there are any complicating factors, although obviously you can never be sure. He doesn't seem to be having any problems, aside from losing his focus when he gets bored. But I think that's just being an 8 year old boy. I didn't realize that achievement testing was an option. His AR scores for reading are around a late 5th/early 6th level, and they test his math pretty regularly - he's around a 5th grade for that as well. Is that the same thing? Or should I look at independent achievement tests? I like this idea far better than the idea of IQ testing, for a number of reasons.
Yes, there are a number of independent batteries, and if you can find a way to get hold of them, proctor them yourself, and then get formal scores after a mail-in... that will allow you the freedom to test 'out-of-level' the way we did.
I highly recommend that. In hindsight, we should have tested DD at about a 4th grade level, not 2nd. But we didn't know. We had no idea that her scores would be
that high. We did it to see how much more time was really needed since she seemed bored and resistant to the 2nd grade material we had, and it seemed like she was running through homeschool curricula faster and faster-- which was weird, since obviously as the material got more challenging, it
should have been slowing her down. It wasn't, because she was more fully engaged the more challenging the material was.
It was a complete and total surprise to us that when we approached her charter school a few months later, they didn't balk
at all about a 3rd grade placement on the basis of those achievement test scores (and a reading level pre-test with them). It really was that easy. Somehow the combination of "out-of-grade-level" and "ceiling" and "recognized, nationally-normed achievement test" was magical.
Jon, for a variety of reasons, while my dad's life was absolutely "interesting" it is most certainly not one that I'd wish on anyone, most particularly my own child. Think Dylan Thomas or Hemingway.

Larger than life, and terrifying intellect, but this world is just not made for some people. Giving it a label is sometimes giving it LEGS, too. There is an element of "you're smarter than me, so I don't want to be around you because I want to feel smart and you'll make me feel stupid, since compared to you, I
am kinda stupid..." that is impossible to overcome when that number stands in for the whole person, too. That's the real danger if the number is VERY high. I never wanted that to happen to DD. Oh, sure, sometimes other parents still feel threatened by her because of the multiple grade accels and what that
must mean... but at least she's never just that number.