Some of these tests are structured to begin with easy items and increase in difficulty until the child is no longer able to answer a series of questions. The assessor may have been trying to test the limits of your son's knowledge. Probably, your son had demonstrated mastery of the knowledge expected of children his age or beyond. Gifted children often reveal that they have knowledge beyond their years, even before they have been taught the concepts or material.
I would encourage you to make an appointment with the school psychologist or other professional who administered your son's test and ask them to explain the nature of the test and how your son performed. That individual is in the best position to identify your son's strengths and weaknesses, discuss his overall performance, and address any concerns you might have.