You're describing what could be a retrieval deficit.
Is that an EF thing or an LD?
It may also help him to learn some typical "translations" for the kinds of questions that teachers ask in class and on assignments and assessments, so that he can re-interpret those excessively vague and open-ended questions into something that works better for him.
I wondered about that, while I was working with him. I told him, for now, just to interpret all questions as "say everything you know" about the concept/term and explained to him that is a better strategy than leaving a blank. This sounds nutty, but DS was astonished. He said, dubiously, "I don't think she gives partial credit." I don't think he even understands what that means.
