What benefits can a multivitim provide to an autistic child?

Probably none. Most chidren, autistic or not, eat an adequate diet and do not require vitamns past the infancy period. If an autistic child does not eat a variety of foods, or eats alot of non-food items, then he might benefit from a multivitamin. Vitamins are not an appropriate treatment for autism.
Controversial . Some feel vitamins have no place in care of autistic kids. However, in light of how well our other treatments are working, it seems worth a try. An option: a physician mother of 3 autistic children developed a liquid vitamin called "agape." one of my patients did settle his episodic agitation. It's expensive, though. See for info & contents: http://www.Awakennutrition.Com/agape/about.Html.
So Far None. I agree that we need an open minded approach, but i would like to see research with proof of effectiveness.There is none.Individual "testimonials" are often biased and very few have turned out to help.See " lorenzo's oil" , a "cure " for a birth defect that was sworn to work, but did not.Megavitamin therapy is another such mishap.At present, nothing has been proven.