Can you actually tell us his book level? There is no reading age 8-10 on AR. AR reading levels are
LY = Lower Years, ages 5-8;
MY + Middle Years, ages 9-13;
MY+ = Middle Years Plus, ages 12 and above
UY = Upper Years, ages 14 and above.
Within these are book bands such as 4,8 or 3.6. Children find them frustrating when at home they are reading David Walliams which on AR is way over their reading level within the AR scheme but clearly are capable of reading it.
AR serves a purpose, children have to read, finish and then answer questions about a book rather than pretending to read a book over a year. The reading and tests are monitored to identify issues or comprehension skills. The impact in primary school is a huge leap in literacy levels and also their quality of work when writing. I was in school as a volunteer when they brought AR in. Each day they read for 30 minutes and are tested and listened to each week.
For your son, he is on the level he is, therefore he should be able to whip through these books, answer the questions correctly and move onto other books. It does not prevent him reading outside of school where he can read whatever he wants. You should treat it exactly the same as reading a book in English Lit, you may not love it but you have to read it as part of what school want.
To show you the range on AR levels, under Middle Years aged 9-13 we have
Dawn Gray's Pyjamas in Space, book level 6.1, points 7
but also
Boffin Boy and the Time Warriors book level 2.6, points 0.5
Huge difference in book level and the difficulty of the book and yet both are in MY level. Book levels start at 0.1 and in primary we tend to have a large number from 4.0 to 6.0 and then much less 6+ to 8.
It is to do with content, complexity of sentence structure, vocabulary etc. Just because someone can read a book doesn't mean they can comprehend what went on, why someone reacts they way they do to situations, predict what may happen, give a synopsis of what has already happened, make inferences and draw conclusions, ask what a word means, you would be amazed how many children read it but don't know what the word means and don't ask me what it means. They just skip over it. I listen to children read on the AR scheme, I have to ask them these questions including identifying sentence openers like fronted adverbials.
I think this is a good example of why a school uses the AR scheme alwayslearningweb.files.wordpress.com/2016/12/improving-comprehension.pdf