Is he reading anything else? Do you read to him/with him at home?
Okay, I'm biased, because my ds was on ORT level 6 aged 5, but that is, I think, simply because reading has always been part of our lives, and when ds was tiny I would read to him, but with him sitting beside me, seeing the words as I read them (I moved my finger under the words as I read), and he really just learnt to read in that way.
So you could try that, read with him at home, but something more interesting (Flat Stanley stories are a good start)
TBH ORT books aren't very inspiring, and if that is the bulk of what your ds sees, it isn't likely to hugely encourage reading for pleasure, and that is what brings reading levels up.
Having said all that, yes, if the teacher is happy, you may want to leave him be. I just get such pleasure out of books, I wanted ds to share that, and he does. He now reads for his own pleasure at least half an hour every day.