An alternative program that helped my daughter with this problem is sound therapy. When it was first recommended to me from her educational psychologist I was skeptical but it has made a huge difference. My daughter went from reading 5 pages a day and feeling exhausted to reading a 100 page chapter book in a day in just 2 weeks of the therapy. That is what I call success! Remember these results occurred after my daughter had acquired fluency in her reading. If the student is not fluent they need to continue a multisensory program based on the Ortin-Gillingham approach to learn to decipher the written word in conjunction with a program that will increase processing speed such as Neurodevelpment through Movement or The Learning Breakthrough.
In the meantime, continue to read to your child. Ask your school board to have his textbooks put on tape or CD so that he can listen to them when he starts to feel fatigued. Without the use of alternative therapies this problem will likely continue throughout his educational career and possibly his adult life. He needs to become proffecient in the technologies available through his school that can bridge the gap in his learning.