When your child finishes the last chapter, have him write a few sentences to describe how the book ended. Your child now has a summary of the chapter book he just read. He can use this summary to write his book report. Help your child be organized, set aside a daily reading time and stick to it. Be sure the book is on your child's reading level.
Many paperback books indicate the reading level on the back cover.
Try to find a book that will be interesting to your child. Familiarize yourself with the book your child is reading. Read it yourself if you possibly can. This will help you to guide your child in the right direction when writing the report.