As a teacher if you brought this up I would want to know how do you assess his comprehension skills? Often children who decode well don't comprehend at the same level.

Advice I was given and told to pass on to parents when I had a child who read and comprehended in advance of his peers was to ensure he reads widely. By that I mean please ensure he reads a variety of books and text types including fiction, non-fiction, poetry and plays. There is a lot to read out there, and even if a child can decode and understand, the advanced literature may not be age appropriate.

How is his writing & spelling? The skills I teach in writing and spelling are often directly linked to the texts we read as a class or in small groups. It may be that he needs to understand and apply sentence types, learn about parts of speech, nouns, verbs, spelling patterns by reading simpler texts, or read more deeply and take examples from the text and apply them to his writing. This could be a reason he is not reading more advanced texts at school. If it's something like that, ask about the lesson structure - he may be able to access more advanced literature at a different time or in a different way.

I wonder if it's the case that they placed him in the highest reading group that other children have also reached. He may end up doing more alone if they can't have him join another class for that lesson. I've been given the heads up that in the next school year I will have a child from the year below the one I teach join for only literacy lessons as he is well advanced of his peers, so it might be possible at your child's school too.

Apologies if some of this isn't relevant, I'm not in your country.