This is a good question, because it is not necessarily the number of books read in a school year, but instead you need to look at the minutes he is reading each day.
For a fourth grader reading 35 to 40 minutes daily is great. Encourage your son to select a wide variety of books on all different levels: some easy, on level, and a few that are more challenging. Also, have him read both fiction and nonfiction: novels, brochures, maps, atlas, manuals, newspapers, magazines, etc.). You may want to think about reading a book together. You can either each have a copy of the book and you read silently, or read it together out loud to read other. Either way you can talk about the characters and events in the story.
Richard Allengton (a reading guru and former President of the International Reading Association) reports in his book What Really Matters for Struggling Readers reported that there is a direct correlation between reading volume and achievement test scores. For example, a fifth grader who reads 40.4 minutes daily is likely to score in the 90th percentile on a reading assessment and will read about 2,357,000 words a year. Compare this to a fifth grader who only reads 12.9 minutes a day. He/she usually scores in the 10th percent on reading tests and reads only 51,000 words a year. That's a big difference.