The obvious simple answer isn't always what he sees. Like when the answer for a descriptive word for a picture ending in "G" is bug but he's confused because clearly its a spider or arachnid and "bug" doesn't even occur to him.
Things like this always bothered me in school too. The problem isn't necessarily divergent thinking. Your son is right here not the school. A bug is an insect and a spider is not. They are not the same. I found many assignments in elementary school to be imprecise to the point of being inaccurate. Just something to consider when he has problems... it might not only be divergent thinking.
