MyHeartBrks:
Wow, what an amazing son you have. And you have been doing what you think is best for him based on your past experiences. That says a lot to me. My son has some of the same interests as yours & almost daily surprises me with the knowledge he has acquired or absorbed (and usually like it's no big deal

).
I don't think my son is nearly as bright as yours is, but his scores on standardized testing have been lower than I expected. I attributed that to the pressure he places on himself as well as the way schools around here handle these tests. (Don't get me started.

)
When he was evaluated by the school psychologist I was happy to hear that the one time DS got somewhat frustrated the tester put that portion away & revisited it the next day. I wouldn't worry about preparing your DS for an evaluation. If you are talking about test-taking skills for school in general, though, 5th grade is a good time to start. I have worked with lots of kids in supplemental education on those skills.
Remember, even though you are not a "labeler" you still know your son best. Your instincts are often right. Just be open to listening to information that is coming in; ultimately, you & your husband will make the best decisions.