Glendora,
Your son's story and your frustration with your family is heartbreaking.
You've already had good suggestions. The only one I could add is appeal to a higher authority. Would your son's neurologist talk to your family about the improvements seen in autism with a Gluten and Casein free diet? Even write a prescription that he must not be given gluten, sugar, dairy...something you can show to grandparents and teachers/schools? This can be done with the gluten sensitive children, and his situation is the same, maybe more critical.
If you haven't read Grain Brain, or seen Dr. Perlmutter's PBS special, look at the other interviews under the media tab and Learn...share articles from the view of helping your son's brain health, not just weight.
http://www.drperlmutter.com/tag/autism/