do you have to use that prompt? He won't understand it, I don't think. Can you ask him to write an essay about a situation he misunderstood? that might be easier.
That's a good one to try. I think I ended up asking him to tell a story about himself that allowed the reader to know him better. Less complex all around. Anytime I give him a writing prompt, I find I have to explain around it more clearly. It's difficult to teach personal narrative and personal memoir to a kid for whom the emotional side of situations are entirely opaque.