The things that can deflate a score could have an even bigger impact if overexcitabilities are an issue. Here's a good explanation of overexcitabilties if you're not familiar with the term.

http://www.sengifted.org/articles_social/Lind_OverexcitabilityAndTheGifted.shtml

Have you asked your son if he has any insight about the differences in the test results? Maybe there's something he can identify that's different about the test taking when he got a high score vs. when he got a low score.