I would like to give my insight on Comprehension. It tests open-ended verbal reasoning (as opposed to Similarities which is very structured and tests verbal abstract reasoning and concept formation). Here are examples of WISC Comprehension questions:
--Why do we have laws?
--Why do we cook food?
--In what two ways is a lamp better than a candle?
--Why are streets numbered in order?
--What does "to kill two birds with one stone" mean?
As you can see, these questions require one to demonstrate goal directed, logical thinking, to be simultaneously concise and complete because responses that are too long are perceived as tangential and responses that are too short do not reveal reasoning. In other words, this test requires adaptive responses. So I would look at whether he has problems with open-ended questions.
