Autism can be caused by immunizations and auto-immune disorders caused by allergies. I would, if I were you take him to a good Naturopath. They can help a LOT. My daughter is borderline special needs, and would be completely autistic if a Naturopath hadn't found celiac disease when she was 3. Meanwhile her old MD (pediatrician) swears she is not Celiac, even though she is. (this is a severe illness where wheat strips the celia out of the intestines)
It's amazing how a Naturopath can help. They are much better at diagnosing, than most MDs. Plus they address the real problem, to solve it, not just prescribing a pill. My favorite naturopaths are Dr. Jodi Berg in Everett, and Dr. Cheryl Wood in Lynnwood. They both will submit you to a battery of tests, to see what is REALLY wrong. They basically cured me of chronic fatigue, and Dr. Jodi Berg saved my daughter's brain from becoming totally autistic.
When my daughter was 3 her brain was shutting down. She talked less at 3 than she did at 2! Finally I woke up and said to myself "something is terribly WRONG!" I immediately took her to my Naturopath, who at the time was Dr. Jodi Berg, and she diagnosed her with a blood test for celiac disease. Dr. Berg also informed me that an MD would have misdiagnosed her as autistic. Within 1 week of taking her off wheat, she started talking again, and her vocabulary increased dramatically within a month. So, my experience is, if you take your son off the offending food, you should see a turnaround within a week to a month MAX. However, in some cases, for gluten, it might be longer, because if the Celia is stripped out of the intestines, it can be a while before they grow back enough to improve the health situation, and the resulting malnutrition can really impact the brain.