I think that this is only a worry if your DS is showing signs of being unhappy with his reading selections. After all, just because he's reading more difficult material doesn't mean he doesn't also enjoy picture books. He may be a middle-schooler by reading ability and interests, but he's still also a 5yo.
For example, my DD6 recently asked us to explore the adult section of the library, and she brought home a few selections, which she reads from time to time in her play room. Yet the books she has piled up in the back seat for car trips are all picture books, which she likes to read aloud, pretending she's teaching a class.
If he's checking out picture books from the school library because that's what he wants to read, I see no problem. If he's doing it because that's all they'll let him check out, that's a problem, and one probably worth getting involved in.
