Should My Little Brother Re-Read A Book He's Read Multiple Times?

Hi. My little brother is in the 3rd Grade and my father is making him read a book. He chose a book that he's read at least three times before, "Diary of a Wimpy." My parents insist that his teacher said that he should read a book that he likes. He's 9 ( turns 10 in September) and I don't think that re-reading a book that is basically like a comic book will help him further develop this reading skills. Also the book contains very little challenging words and lacks any type of story-line. So my question is, should he re-read this "book" again or should he read something more challenging? Also is this book on this reading level? Thanks.