How do you deal with a severely autistic child's behavior when he becomes violent?

Medication. Medication can be helpful to take the edges off of violent outbursts, and some success has been found with vests that give the sensation of a hug to select patients.
Medical? I hope the child is receiving behavioral intervention. Abilify (aripiprazole) and Risperdal are approved for treating irritability in 6-17-year-old children with autism. Along with oral tablets for daily use, they do come as " melt-tabs, "little sponges that melt on contact with the child's tongue and work really quickly. Straight talk about psychiatric medications for kids by dr. Tim wilens is a good book.