I think his problem comes down to processing time. I doubt he is too "dumb" to learn the assignments, just does not process information quickly enough to react to what is happening in front of him.

OODA Loop
The phrase OODA loop refers to the decision cycle of observe, orient, decide, and act, developed by military strategist and United States Air Force Colonel John Boyd. Boyd applied the concept to the combat operations process, often at the strategic level in military operations. It is now also often applied to understand commercial operations and learning processes. The approach favors agility over raw power in dealing with human opponents in any endeavor.