@Zoommeout as a parent we faced this dilemma a couple of years ago but with a much worse starting grade. Ds went from a 4 to an 8 but he was 9s pretty much across the board. So as a parent I will tell you what I did because like your son, mine was very willing to work at it and got an 8 in English lit in the end.
To help him you need to understand what gets marks, I am going to assume he is doing AQA as that is the most popular exam board for lit. Open up the mark scheme to any of the past papers online and read it. ie like this one https://filestore.aqa.org.uk/sample-papers-and-mark-schemes/2022/june/AQA-87022-MS-JUN22.PDF
Most importantly learn about Assessment Objectives, AO1 - AO4, he has to hit every box.
AO1 is Read, understand and respond to texts ie use textual references, including quotations.
AO2 is language, so he needs to analyse the language used
AO3 is the context in which it was written ie what year and what was happening at the time
AO4 is how they write it, language, structure of their sentences.
Then there are levels a 6 is the highest and a more full description of what they are asking the markers to identify is listed. Pull up the corresponding past paper question paper and then find the Shakespeare play your son is studying, look at the question and extract and then look at how they list out different bits under the AOs. For Ds it was Romeo and Juliet, so say they ask about male violence and of course they give an extract, could your son literally mentally think his way through the text pulling out all the times this is portrayed? R&J opens with a brawl in the streets. Ds did not do the extract first, he did the first time they encounter violence, worked his way through to where the extract falls and then finished the essay with where the violence ends, ie Paris is killed by Romeo. They write a lot about a little, ie they explore the scene or language used not 8 times the violence occurred.
Everything they study from the novels to the poetry is fiction, an author made deliberate decisions about the language they use, especially when it can have more than one meaning or foreshadows an event.
I can highly recommend Mr Salles both on youtube and we bought his books for Romeo and Juliet and A Christmas Carol, they helped immensely plus they have grade 9 essay examples in and were well worth the money for us. Re poetry, again Mr Salles, especially the 5 key quotes, Ds knew how he would start any essay on the poetry, ie at the beginning of the poem and talk about form and structure. The CGP revision book is great for this, breaks down every poem and again all those AOs.
Practise looking at unseen poems, what does he think they mean? There are lots of comparison ones on google that have been used for revision in other schools. Fundamentally he needs to know the texts, what happens when and he can embed quotes, ie his abusive language towards Caliban, eg ‘poisonous slave’, ‘hag-seed’ you don't need a whole sentence, just the key words.
I hope this helps.