Hiya! Ex primary school teacher here! It would depend on what phonics scheme they use, as to the books that they send home. As you mentioned about ‘simple’ books, these are still around definitely; schemes tend to start with picture books and then increase in difficulty as the children go up the ‘levels’. If you feel they’re too tricky for him, definitely mention it to his teacher and they can review with him! Also, however, if you do want to read additionally with him at home, you absolutely can do that too, and it can be really disheartening for children to try and read books that might be too hard for them ☹️ Hopefully his teacher can shed some light on why he’s on that level of book!
I will say that as he’s now in Year 1, the books are definitely trickier than in Reception, however should be matched to his phonic level so that he’s both learning to segment and blend, and develop his comprehension!
Following Im with you on that, my sons in reception and i think the books are quite hard they send home!