Take him to an integrative medicine doctor or Naturalpath who can work him up for Lyme, Bartonella, Babesia, heavy metals, mold illness, chronic viruses, and leaky gut.

See this link..
https://www.scientificamerican.com/article/is-it-possible-to-recover-from-autism/
or
https://www.spectrumnews.org/features/deep-dive/children-who-leave-autism-behind/
or
https://healthcare-in-europe.com/en/news/some-children-can-recover-from-autism.html