Hi OP, I am assuming he is finishing year 9 and going into yr 10?
And do you know (probably not but hey) if that prediction of 4 is "this is what he would get if he sat the exam tomorrow" (fairly useless imho unless given in April of yr 11) or is it "this is what we expect him to achieve in two years time if he continues as he is now.
Assuming the latter (more useful) then he has two years of school to improve. If I were you I would have a look at the AQA GCSE Eng lang mark scheme (don't do past papers, toooo early) and see what sorts of things he needs to have mastery of.
For instance - is he all over the parts of speech, does he know what a metaphor is, a simile, alliteration? And more to the point, does he know why they are used and what effect they have? If not, that's a good place to start. Find some texts, maybe things he knows, passages rom a favourite book and start to analyse what effect the author is aiming for and do they achieve it. Look at the structure and devices used there. Try to find another passage and comare the two.
And make sure he reads, reads a range of texts - try discussing what he is reading - can he verbalise anything about it? It's skills he needs to be acquiring just now.