I have gone through the very same experience with my 16 year old daughter, and reached the same conclusion as you. She is so smart but you'd never know it by looking at her grades. She has always been quirky and "different" than her peers but she did well all through elementary school so I didn't dwell on it. She started having issues in middle school with her school work (not completing assignments, especially homework, and lying about it) but we could usually get her back on track. But it all spiraled downward in high school, and she just finished her sophomore year and failed her GT World History class and didn't do very well (although she managed to pass for the year) in her other classes. Part of her problem is that she doesn't see how some of the things she's being forced to learn have anything to do with her chosen career path (arts related) -- her thinking is that since she's not going to be a scientist, so why does she need to take 4 years of math and science? And she does not relate to her peers at all and has maybe 2 friends. She absolutely HATES the whole high school experience -- too many kids, too much teenage drama, etc.
So we made the decision to put her in a small private school because we feel the more intimate, hands-on environment is better suited to her needs. Financially this is going to cause quite a strain, but we knew that keeping her in her current environment was not going to work. She is going to summer school at her new school to make up lost ground. She just started, so we'll see how it goes, but I'm cautiously optimistic.
Regarding the testing, his school is required to do testing if you want it, but they will drag their feet. I have been working with our school district's special services for months and have gotten no where. I asked for the "full evaluation" and was told they don't do that. The woman met with my daughter for a short time and says she isn't ADD . . . really? You can tell that from just one meeting? I don't think so. She was also supposed to arrange an in-classroom evaluation of my daughter which never occurred before the end of the school year even though I repeatedly reached out to her. So of course now we're withdrawing my daughter from the school so that's the end of that, and we can't afford to do it on our own. But you know what? I'm OK with that. What does it get me, anyway, to have validated what I already know? I'm not going to put her on meds, that's for sure. We'll work on it through behavioral adjustments.
Sorry I don't have any real answers for you . . . I just wanted you to know I understand your situation and how you feel . . . and it's an ongoing struggle!