I'm sure that the testing gurus will speak up soon. It looks like your son "hit the ceiling" on the CoGat but there is no way of knowing how much farther he could go given the limits of the test. Obviously, the school believes that CoGat and ITBS give them different information but I don't know the relative strengths of the the tests. Neither CoGat nor ITBS is considered an IQ test. What kind of IQ test did he have in first grade? You may want to review the inventory of tests on the Hoagies website http://www.hoagiesgifted.org/tests.htm

I don't think that it could hurt to appeal. I would talk to the school about what kind of additional information they would consider before you run off and spend $ on private testing if you decide to go that route.