Of course that's not the whole story. He's relatively good, for example, at "rationalia"; he can solve fairly complex problems that he cares about. This "reasoning" capability is now thought to be fairly distinct from IQ, this also explains why some very smart people make very odd choices.
The "cares about" part is relevant to two recent stories.
One concerns shoelace tying. We'd made very good progress, but then he seemed to lose interest and regress. So I was preparing for a new attack when, one day recently, he started negotiating for a book he wanted me to buy him (he's a bibliophile, sometimes he even reads them). I jumped on it as a reward for what I estimated would be a 2-3 weeks of practice to achieve a goal of 5 successful ties on two on-foot shoes.
My son instead insisted he could do it immediately. Skip the training - go straight to the test. I demurred; I feared he'd get frustrated and meltdown. We worked out the details and, of course, he immediately completed the test and got the book.
The second story concerns my locked down iPhone. He's shouldn't to be able to access the web browser. Except that he discovered a back door through a generated hyperlink within a Twitter app he uses that gave him an embedded browser view he could manipulate. He's not supposed to be smart enough to hack my bleedin' phone.
I don't think his IQ tests are wrong. I do think there's a bit they don't quite capture.