Welcome!
It's comparing apples and oranges. One is an academic achievement test, and the other's an IQ test. Assuming a child without testing issues such as perfectionism, who had an okay and representative testing day for each type of test, it'd be possible to score high on an achievement test but not so high on an IQ test if the child were simply academically advanced, i.e. a bright hard worker who had been properly stimulated. It'd be possible to score high on an IQ test but not so high on an achievement test if a child had been relatively undereducated up to that point, compared with others with roughly the same potential.
I think it's also possible for a very bright child to score relatively poorly on a particular test due to divergent, creative thinking. The Wikipedia page on Richard Feynman states that he achieved an unwhopping 125 on an IQ test, surely an underestimate of his intellectual ability no matter how one views the accuracy of IQ tests.
I have no wisdom. But if I were to pretend to have some, I'd advise you not to stress. I think you might want to consider trying a portfolio first, unless the money for testing is no big deal for you. A while back I put together
some info on DYS portfolios that may be helpful. For example, you should realize that you don't need to send past academic work samples; you can have your son generate something new as a work sample. Plenty of people have had their children speak on video about topics that interest them, for instance.
Read the threads linked to from that page and you may feel that you have a better chance than you thought of gaining entry for your son with a portfolio. And the consensus seems to be that you don't prejudice your child's chance of entry by submitting information resulting in a "need more info" temporary denial; that is, if your portfolio submission is rejected, you'd probably have just as good a chance with testing afterward. Good luck.
