My DD is in a comprehensive with mediocre results, and was assessed at the beginning of Y7. I can't remember the numbers she's in, but she is a good reader. In the first few weeks of Y7 she brought home every Diary of a Wimpy Kid book in succession, which she read in a night and took back. She had read most of them 3 yrs earlier anyway.
She then worked her way through the Jacqueline Wilson books that she actually owned anyway, and again read them very quickly. She won a prize at the end of the first half term for reading the most words, but in some cases she just took the book out, did the quiz and didn't bother reading it because she already had.
She kept telling me there was nothing else, and I was all ready to go in and complain, but then once she made some new friends who shared her love of reading, she got recommendations from them, and then moved on to books like Noughts and Crosses, The Hunger Games, Gone etc. She hasn't been moved up a level, she was just rejecting those books because they looked "scary" or too big and grown up.
Have you actually seen the library? Do you know he's not allowed to borrow these other books? DD reads books that we buy, or get from the local public library and she can still quiz them and count them as her reading book.