I wouldn't assume all the books will necessarily be under his level. Many high quality picture books have a higher reading level than "early readers" or simple chapter books because they are meant for parents to read to children vs children to read to themselves. Also, the library probably has non-fiction, which can provide new content even if it is not necessarily advanced reading level.
I always find it interesting to see what my DD brings home from the school library (her school is K-6 though). Sometimes it is easy readers and simple chapter books and sometimes it is encyclopedias. She's gotten books on magic tricks, how the postal system works, cookbooks, etc.
She can bring in her own books from home as well, but she does not always think to take them out of her backpack.