Could anxiety have caused some of the lower scores? For instance if he was nervous, it would be difficult to hold a string of numbers in mind and repeat them. I would make a game of it and ask him at home. Make up a string of numbers and see if he has trouble remembering. You didn't say how old he is but maybe if we knew that we could give you an idea of what is average (for instance for a 6 year old, average might be a string of 4 numbers, for a 12 year old it might be more like 6-7).