... he can get books from the library only once a week, and there's a limit on them.
In our experience we found that the circulation of books through school libraries may be somewhat constrained by the amount of time which volunteers are able to offer, to help check-in and re-shelve books. This can lead to limits on the number of books checked out by a student, as well as limits as to how frequently a student may check out books. Do you have a public library nearby? That may be another option for more frequent book choice, and also getting books in higher quantities.
... We also wonder what the school is giving them to read, for he frequently comes home with such gems as "Dinorella" (a dinosaur-themed retelling of "Cinderella"- where every word began with the letter "D".)
Have you asked him about the book? Was it entertaining? Did the whole class get the book to read while they studied the letter "D"? Was it assigned to one reading group? Did he freely choose it from a selection of books? What made him choose it? Was it because another child read it? Did he like the cover? This could be a wonderful conversation. It might also be fun to read the different roles together. Or write about something else he may like, using mostly words beginning with one letter. Our family found these conversations invaluable.