Have an additional idea, "What's missing" is timed, and it is the very sort of task that lends itself to over-thinking, particularly if you are strong visually and have strong attention to detail (like a strong chess thinker would in your son's case.)
Here's an example of that sort of question:
http://www.testingmom.com/free-stanford-binet-practice-test-questions/And thinking that can lead to missing it:
"There's an airplane... what's missing? A pilot, passengers, a door, is this in the air or on the ground? if on the ground, then the wheels are missing, of course the wing is missing, but that is SO obvious, maybe they removed it so I can see that the wheels aren't there. Where's the tail numbers? Also, is the shading correct with the blue above and white below? Is that shading or paint?.... what? I'm out of time...?"
