I'm wondering if, based on the 75 he got, you could tell the school you're concerned about a learning disability and ask them in writing to do a full educational evaluation including an IQ test with WISC IV.
I think it's obvious the 75 is wrong, but if they're willing to exclude your high achiever based on that one test that they take so seriously, you should consider playing their game and take it seriously too. Operate as if it's really true and ask why he's not receiving the special services he obviously needs. Push for the evaluation based on the "alarming" test results. I think they'll end up telling you the results were faulty, and if that's the case you probably have another entry into the GT program. But I also think you're entitled to a full evaluation by the school.
I have experience with this kind of testing foul-up. One of my boys got a 97 on a GT group test and when they did followup testing for GT on the CoGat, his scores averaged a 20! When we asked him about that test he told us he was in a hurry to get back to class so he didn't miss lunch. He wasn't accepted to GT based on that. Fortunately, we called the GT office and asked if they had an explanation for how he lost most of his intellect between the two tests, and they agreed to a different test at a different time (he was accepted).
