There are quite a few threads OP in secondary ed which will be very relevant - try searching revision or revision techniques.
What board is this? Does he have the study guides? They are very good, as are some of the YouTube videos.
How does he prefer to revise? It's best IMHO if it's active - you tell him a theme in Macbeth and he tells you all he knows about it, for example (having first made the revision cards or mind map).
Jekyll and Hyde oh man! My DD studied that for her final year of her eng Lit degree! Still, GCSE is looking for a more basic comprehension of them themes.
It's useful to learn quotes but they don;t need to be long and the key is showing their relevance. Ape-like fury is a good one for J&H - relating to the character of Hyde, man as a beast etc. Or for the character of Lady Macbeth (for example) - she talks about wanting to be filled with "direct cruelty" and talk about the significance of that. Those are literally a couple of examples from the top of my head but you get the gist.
The first thing may well be to have the timeline of the play/novel clear in your head (tricky with J&H but still) as the structure is important. You don;t want to be telling the story, but OTOH you do need to be able to talk about the extract in a meaningful way and what happens before and after.
There's a lot more to say but as I say, have a hunt on this board, there are some excellent and knowedgable posters