I know you don't want to hear it, but there is probably a good chance it is a microbial infection of sorts.
http://en.wikipedia.org/wiki/Causes_of_autism#Viral_infection
Many studies have presented evidence for and against association of autism with viral infection after birth. Laboratory rats infected with Borna disease virus show some symptoms similar to those of autism but blood studies of autistic children show no evidence of infection by this virus. Members of the herpes virus family may have a role in autism, but the evidence so far is anecdotal. Viruses have long been suspected as triggers for immune-mediated diseases such as multiple sclerosis but showing a direct role for viral causation is difficult in those diseases, and mechanisms whereby viral infections could lead to autism are speculative.[22]
http://www.nature.com/news/bacterium-can-reverse-autism-like-behaviour-in-mice-1.14308
Bacterium can reverse autism-like behaviour in mice
The work builds on previous research by Paul Patterson, a neurobiologist at the California Institute of Technology (Caltech) in Pasadena. In 2012, he and his team created mice with autism-like symptoms by injecting a chemical that mimics viral infection into pregnant mice; those animals then bore offspring that were less sociable and more anxious than wild-type animals2. The autistic mice also had 'leaky guts', in which the walls of the intestine break down and allow substances to leak through. Several studies have found that humans with autism are also more likely to have gastrointestinal disorders, suggesting that the two problems may be linked3.
“It’s incredible that putting this one bacteria back can reverse all these widespread changes,†says John Cryan, a pharmacologist at University College Cork in Ireland. Although many anecdotal reports and small studies have suggested that ‘probiotic’ bacteria, such as those found in yoghurt, and antibiotics can help with the symptoms of autism, Cryan says more research needs to be done. Because there are a number of types of autism in humans, it will be important to look at how different symptoms might be affected by different microbes. Another question is whether the microbiomes of the mice — whose symptoms result from maternal infection — differ from those of mice that are genetically predisposed to autism-like symptoms, Cryan adds.