As a parent have you looked at any of the past papers and mark schemes for English language? If you can see what is needed then you can help him get a higher grade too. Literally walk through each question one by one and look at what the mark scheme is looking for. Once you have done one or two it should be easier for him to pick out what the examiners are looking for. Ds found it easier to learn this from me rather than a teacher in class.
If he has to do a narrative or story they are usually given 4 or more generic titles to choose from. He doesn't need to think of a story on the spot, he can prepare rough outlines for stories before the exam. That means you and him sitting down and working on descriptive words for things, the best ones are weather, you can always stick weather into any narrative. Google will happily provide good descriptions for things. The same for describing a person. He is 15/16, they are not looking for a best selling novel, just how they use language and punctuation. My sons didn't do AQA for lang which is the usual choice but both went in with outlines for 3 stories that you could fit to a title, like The Choice, The Mistake, The Playground. There are lots of videos on youtube about writing narratives too. Mr Salles, Stacey Reay.
From Stacey Reay (teacher of the year) Fronted adverbial ie Frantically, cautiously, Wildly, Enthusiastically, Accidentally, Without warning, Within seconds. Use outside/inside - she greeted every person (but felt like a museum exhibit) or abstract nouns such as anger, indignation, outrage, courage, daring, bravery, integrity, virtue, principle etc. I really hope this helps. Mine both increased their English grades with the above. As your son, like mine, has 9s in other subjects this seems like it is an exam technique situation.