Yes, if you think the challenge program is what he needs you should definitely advocate for him! First, I would demand that the CogAT be rescored properly. The age percentile means that is where he ranks among kids of that age. So, at age 7, he did better than 70% of 9 year olds on this test. What you want to know and what they should be looking at is how he did compared to other 7 year olds.
Second, you need to find out exactly what the entrance criteria for the program is, and every school district does this differently. My own ds scored poorly on the CogAT but extremely high on the ITBS and as you said this is unusual. The CogAT is an ability test while the ITBS is an achievement test. So, what you would expect to see is congruity between the two scores or, in the case of an underachiever, a high CogAT and a low ITBS (high ability but low achievement). This is really what they are used to seeing when the two don't match up. However, as pp noted, the CogAT does not reward the creative or outside the box thinker. Those kids are often more highly gifted but score poorly on the CogAT because they don't give the expected answers.
We are lucky that in our school, the GT teacher takes identification very seriously. She talks with teachers, examines the kids' work, and gets to know them one on one. Thus, she was able to recognize that the gifted program was appropriate for DS. And he has definitely flourished since he started it this year, so it is clear that her assessment was correct and the CogAT scores did not accurately represent his abilities. Unfortunately, not all school districts take the time and resources to do this. I do hope that yours does. But I would definitely push them on this in order to get your son the education that is appropriate for him. Good luck!
