omviki,
welcome from a fellow Schaumburg parent. DD is in second grade and will also be taking the CoGAT. She also took it last year but just missed the cut off.
As SPG said, there is no way to prep your child. In third grade I believe the test is timed. I would ensure he is well rested and has a good breakfast. They give the test over 2 days. I would tell him it is a bubble test and I don't think most kids have seen this before. I would also tell him to try his best.