I’m an English teacher and we use AR.
Zone of Proximal Development is a theory of Vygotsky’s - basically it’s the level at which you can learn most readily, without either frustration or boredom but whilst still being challenged. AR have used this idea in their programme. Each book is given a level, and pupils are given a recommended range of levels which should be appropriate but challenging for them. The pupils shouldn’t be able to work out their reading age from the ZPD as that could be really demotivating for them, but the ZPD will be updated as they improve.
When a pupil finishes a book they’ll do a quiz on it and be given points etc. AR will also calculate how many words they’ve read and the average amount of time they spend reading based on these quizzes. It’s a good way of motivating pupils as well as keeping them accountable.
Your child will be completing 4-6 Star reading tests this year so don’t panic yet. Help him choose books (AR will give him suggestions within his ZPD when he logs in and the AR Bookfinder search engine will help to conduct more advanced searches), discuss them with him, and aim to have him reading for about 30 mins per day if he’s not already doing that in school.
If I were you I’d ask the school if they were going to provide me with updates on his Star tests through the year - then you can see if he’s making progress.
Bear in mind that reading comprehension consists of 3 things - decoding, vocabulary and background knowledge about the world. At his age the first is probably less of an issue, so work on developing his vocabulary in day-to-day conversation and help him to improve his general knowledge.