If you browse the forum, you'll find that many people have had similar experiences.
The CogAT is normally scored by age, so the grade skip should not have affected his standard scores/percentiles. However, it is not uncommon for children with high cognitive ability to perform lower than expected on the CogAT. This is why the publishers carefully call it an aptitude test, rather than a cognitive/intellectual ability assessment (despite the name).
If a WISC-V (latest revision) would potentially provide him with access to more appropriate services, then it may be worthwhile to your family to have one administered. BTW, it's not that the age 6 WISC-IV is no longer valid, more that it's not accepted by this program for admission. It is also true that scores from early childhood are less stable than those from his current age.
