Has anybody mentioned Autism, I have a 7 yr old girl who is like this and has been diagnosed with Autism. She has very violent outbursts as she doesn't understand the world around. You need to get in contact with the national autism society and also parent partnership who will be able to give you advice how to go about a diagnosis.
Also in the meantime try giving him fish oils with omega, I give my daughter this and it really does work to help calm her down. Also ignore the bad behaviour and reallypraise the good behaviour. Does your son sleep at night? I ask this as many autistic children don't sleep at night until it is completely dark.
I hope this info is of some help.