Firstly I'm NOT an English teacher and neither are my DCs this age.
This is what I'd do. I have a house full of ADHD.
Forget about the actual writing for now.
Forget about the second story option/ question.
Focus on the descriptor question.
Get a whiteboard, put it on the wall and STAND UP to do the work. This is very important.
Practice writing ideas/ key points in spider diagrams, lists or whatever.
Gradually start to turn these into paragraph plans.
Sometimes just do vocabulary lists.
Then do this on paper/flip chart style.
Only then, move on to essay writing.
Do this over several WEEKS.
Practice doing this verbally too... See a picture in a magazine? Get him to run through the process. On a billboard? Ditto. Take a still from an advert on TV. Same. Use your own pictures/prompts with the whiteboard.
For now, completely remove the 'sitting at a desk with a blank piece of paper' anxiety. Develop skills and confidence in the meantime. And then come back to it.