It appears that your son has just barely been keeping his head above water this year. A big part of his problem sounds like weak organizational skills. The first step to improving these skills is to get him to write down his assignments in a notebook. The next step is to sit down with him at the start of his regular study time and help him organize his work. Then, get him started on the more difficult assignments by making sure he understands the directions and has a plan for doing the work.
Besides improving your son's organizational skills, you want to be sure he doesn't have a reading problem. In the fourth grade, the emphasis in reading has shifted from acquiring basic reading skills to reading for information. Children who have mastered the mechanics of reading stories may not know how to dig the meaning out of their textbooks. For this, they need skills in determining cause and effect, finding the main idea, interpreting content, establishing a sequence of events, and sensing relationships.
Reading in fourth grade has also become more challenging because of the need to acquire a special vocabulary. Earlier, the children knew the meaning of the words in their readers. Now, they must learn the meaning of new words such as symmetry, culture, and gravity.
Now is the time to make sure your son has the organization and reading skills needed to handle his work. Without these skills, he will continue to struggle in school. Also, keep him interested in reading for fun by reading with and to him. The more your child reads, the better he will read.