@DottyLS this was my Ds2. 9s across the board including History where he often got 100% but he scraped a 5 on his November mock for English language. In the end he got a healthy 7 and an 8 in English lit. The rest were 9s. It was definitely exam technique for Ds. They diagnosed him with a processing disorder. I think the boy just won't waffle, if he doesn't know the answer, he wouldn't guess.
What helped us - pull up the mock paper if you know which one he sat. If not just pull up the correct exam board, any past paper and pull up the corresponding mark scheme. Mine sat Educas most sit AQA, we printed out the resource sheet so he could physically highlight the text like he would in the exam. Then go through each question one by one. Identify what gets marks. On Ds's it said list 5 things we learn about Rose in lines 1 to 15 but there was always at least 8 things. Look at the mark scheme specifics on if you have to rephrase an answer rather than writing verbatim what is in the text. Look at what they say don't award marks for. Go through this with him, help him identify things in the passages.
Once you have gone through one set of papers with him ie Summer 2018 papers 1 and 2. Pull up another set and see if he can now answer the questions himself but again go through one by one. Answer, check the mark scheme. For any narrative they have to do, completely agree with teen that they memorise set scenes, mine were given house, forest, beach, rainy day, sunny day and some outline stories that can be applied to the usually generic narrative titles. Also use an online thesaurus to substitute mediocre words for better ones. And punctuation, they are usually looking for how you convey something not really what you are conveying.
For English Lit (and Lang if AQA) Mr Salles on YouTube, also Stacey Reay for the books and poems. They are 16, they aren't writing anything groundbreaking, they just need to know what opinions to regurgitate and how to structure it.
Examiner's reports lay out where students excelled, where they went wrong and what they need to do to improve. There is plenty of time to nail this before actual GCSEs. Be aware in the future that some uni courses want a 6 in English Language, I don't know how many and it may be specific to what Ds2 is looking at but I thought I would just tack that on. And good luck.