As already mentioned in another reply, Omega 3 fatty acids are crucial along with fixing the gut with probiotics. You may have heard of leaky gut at some time - well there appears to be a similar condition called leaky brain. The junctions that are supposed to protect the brain are opened and things that are not supposed to get in somehow do. We are still studying this phenomenon but it seems to be associated with inflammation.

Gluten free and dairy free diets as already suggested are helpful. Gluten and dairy cross react with each other so this is why both usually need to be tackled at the same time. What is critical is no vaxxines and no MSG or its cousins in the diet. Consider NAC. It increases glutathione which can help clear the brain and it is a glutamate modulating agent. Glutamate is often very elevated in autistic children. Here is a study that used NAC to treat children with autism. Dosing is given. N-Acetylcysteine in the Treatment of Childhood Autism

Good luck