This is such an old topic that I'm puzzled to see it appear yet again. I've read several books about cow's milk in human history, society, and nutrition, as well as going over the research on cow's milk (in particular the protein casein, the sugar lactose, etc.) in autism spectrum disorders.
The results to date: dairy products do seem to exacerbate some symptoms in a subset of autistic children within the confines of a limited number of simple studies. It may be dairy allergies, hormones or other results of factory farming, or something else. No one knows (I checked PubMed via the National Library of Medicine about this issue a couple of months ago).
No researcher claims milk or other dairy products causes autism, because no research demonstrates that it does, and there are autistic children coming from dairy-free homes (all it takes is one autistic child from such a home to disprove the hypothesis). The same holds for the vaccine/thimersol hypothesis, already disproven.
But you can try eliminating dairy from your diet and see what happens. As the posts here suggest, probably little if anything will happen one way or another.