Way late with my comment, but I'm not sure it's doing anything more than making him be able to do math problems while juggling or doing other exercises.Ken's video shows some great work with mental work done while under physical pressure.
I base this mostly on my experience with snowboarding, which was impossible at first, then became something I could do while daydreaming. And now, I just daydream about snowboarding (a different problem altogether).
The brain develops some pretty specialized circuitry to handle "routine" processing. The more specific tasks are practiced, the more automatic they become. If math lives in a very specific part of the brain, it's not going to help verbal, or spatial recognition. Task specific training will continue to be more effective.