WARNING!
I would strongly advise against prescribing B12 to children without first consulting both your GP and a local medical clinic. B-12 (or methylcobalamin) produces the highly toxic methylmercury as a part of it's action. Methylmercury causes neurodegeneration and, by strongly binding to proteins, is highly toxic towards tissues. The organomercury compounds are best left in the cell tissues where the body can isolate and deal with them. The only reason methylcobalamin is used in medical treatment is to help improve the strength of the peripheral nervous system for the disorder peripheral neuropathy. The best thing (I speak from both scientific research and personal experience here) for anyone with Autism is a wide and varied lifestyle, a good TLA or other support staff in school and positive attention. There is little else that you can do. This support will often help autistic children develop compensatory methods for dealing with problems. Speak to a local autism support trust, and they will help you more. Be careful not to get dragged into expensive (and more often than not, bogus) medical treatment, as a great majority of them are completely irrelevant.