Begin by getting a list of the most frequently used words in reading from your son's school or a library. Then slowly teach him a few words at a time in this way. Have him make a set of alphabet letters out of cardboard or sandpaper. He should make extra letters for those he'll use often. Have him arrange the letters in sequence for a word. He should also trace the letters with his fingers and write the word. Slowly, he will begin to improve his recognition of basic words. While you can help your child, he probably needs special help from his school in order to handle the more sophisticated reading required in middle school.
Make an appointment with the middle-school counselor and ask to see your child's standardized test scores. These scores will give you a picture of how well he has been reading each year in school. If your son has been reading one or two years below grade level, you should talk to the counselor about having him tested further. There are many reading problems that are not diagnosed until the upper grades when reading requirements increase.
Once the reasons why your child is having problems with reading have been identified the school should be able to put in place any supports that he needs, such as books on tape, a resource teacher, a note taker, or extended test time. These aids will make it possible for your son to keep up with his classmates.