Have you ever had your son checked for vision problems. My daughter has recently been diagnosed with convergence insuffienciency. She has had difficulty keeping up with other students especially when reading is required. She was tested last year by an opthomologist and was found to have near perfect vision. I had a niece who was diagnosed with this same thing and she has gone through therapy and has made major improvements. She was always considered ADHD, but she actually had a vision problem. This is how I knew to get my daughters eyes checked by someone who specializes in how the eyes work together. My daughters left eye does not follow along with her right eye on an object which causes her to see double and she also has difficulty in following words in a sentence. Most eye doctors do not check for this in their eye exams. Just search about eye therapy and you can get a lot of answers.
Are you dealing with anxiety? Children suffering from anxiety have difficulty learning in school as their brains are constantly absorbing all of the sensory issues in the class as sounds, sights, movements, even feelings. Just a thought ...