I like to remember that Richard Feynman reportedly achieved a whopping IQ score of 125. It's obvious that IQ tests measure some aspects of behavior related to intelligence, but they don't capture everything.
Your son's what, seven turning eight, and working at an eighth grade level based on self-motivation? I think that the numbers you've gotten are probably an underestimate. Bright but motivated children just don't usually achieve that way, although I guess the recent post by Bostonian about prodigies shows that I shouldn't assume very much.
I'm sorry you didn't get the results yet that you expected, but I wouldn't give up on DYS at all. I would bet there are plenty of DYS admittees with far lesser achievement at the same age, and a fair number with a similar profile that were admitted.
