I just have a second, but I am in Colorado as well. The state is pushing to have more standardized identification criterion from district to district so I can tell you generally what you're looking at needing.
He didn't need to be ided in 3rd grade although a lot of districts give all of the kids the CogAT in 3rd and use that as the "ability" piece. He'll need both an achievement score in the 95the percentile or above in math (his MAPS will work for that) and an ability score in a quantitative area in the 95th+ or some other thing such as a behavioral scale (SIGS or similar) or there may be other options. Basically, you need indicators in two different areas, so two achievement tests won't work.
Our local districts won't use composite ability scores (only the subtests) and while they did use the Perceptual reasoning subtest of the WISC for my oldest as the quantitative ability score, they won't use that anymore. My youngest had a perceptual reasoning index on the WISC in the 99th percentile and achievement scores in the 98th for math as well as a composite score on the WISC in the 99th+ range and they would only use the Verbal index for a reading ID combined with achievement data.
The CogAT may be your simplest and cheapest way to get the ability score but it doesn't work well for all gifted kids.
