Hi,
Also consider how he does in test situations and who the tester is.

For example, does he have a bit of perfectionism in him, where if he's not sure if he could get it right he won't try. Perhaps he needs a lot of encouragement?

Or, were the first set of questions so easy (with no explanation from the tester) that he was offended by them. Some testers will explain to the child that, oh these are so easy, I know you know them, but could you do me a favor and just answer them, then I can give you the next set which would be more fun ... or something like that.

Then, when the test starts to get hard, the tester would say something like ... wow, you're doing great. Now, this next set of questions ...you're not suppose to know how to do it. But do you want to just try and take a guess. Just for fun...

etc etc to coax the answer from him

Not sure if this is happening with your child, but just a thought...

goodluck!