English teacher based in South Derbyshire here (so your son could end up in my actual classroom!) In Y9 we begin the GCSE syllabus and essentially have a dry run of both English Language and Literature GCSE over the course of the year. For the GCSE English Language he would be expected to be able to write creatively for one paper and to write a strong argument for the second paper.
To that end, I would be exploring figurative language devices with him (you can look at this through the medium of the fiction texts you read) so similes, metaphors, personification, sound imagery etc. Maybe get him to write short extracts describing the sea, the weather, someone’s face etc. in a creative (as apposed to factually descriptive) way.
Also explore with him how to successfully argue a point. In a nutshell, students are given a statement in the exam and asked to argue for or against it in a specific format (letter, newspaper article, essay, leaflet or speech) e.g. Homework should be banned. Ask your son his opinions and explore how to successfully get them across with maximum impact. Essentially explore persuasive devices with him (direct address, repetition, rhetorical questions, facts etc).
Finally, where students lose marks is a) lack of sentence variety and b) lack of variation/accuracy in punctuation. Practice different sentence types with him (beginning sentences with an adverb, short ‘punchy’ sentences, complex sentences with embedded clauses etc) and make sure he can use the higher level punctuation marks accurately and effectively (colon, semi-colon, brackets and dashes)
This will set him up well for the demands of GCSE which he is likely to begin exploring in Y9.