It does seem a shame for him to be so limited during the week.
A few ideas to eek out the books over a week, but perhaps you do them already.
Obviously predicting the story is out as you know what is going to happen next, but can he re-tell the story? Act part of it out? Especially as he knows them so well. First with pictures to help, then without.
Can he find particular words on the page, ie a frequent word or a characters name? Could he then spell them out loud first by looking at it, then with his eyes closed? He should get better/quicker at this over the week.
Could you say a word from the story and then he has to say a word that rhymes, this could lead to silly, made up rhymes.
I would make these things into a game rather than a test, so perhaps he asks you to carry out some of the above as well. Or you take turns reading the pages so he gets a chance to hear the story read with fluency and expression. I would then encourage him to really focus on the expression, so he really has to act 'over the top', you could even occasionally record him and then he can listen to himself read.
If all these things are easy then perhaps you ask for him to be moved up a level but first i would try to be creative with the book and ask the teacher for some other ideas to bring the books alive. Also, although reading schemes help children to feel familiar and safe with story langauge/characters etc, i think they can limit a child's reading experience. Take some non ORT books from the library - this will help him get used to other fonts/styles of text, story lines etc. Hope this helps.