Please consider eliminating the possibility of food sensitivity.
I am personally aware of a number of cases, where children exhibited violent or suicidal behavior which were caused by reactions to gluten, dairy, sugar etc. and in one particular case tomatoes!
Note:I have trained in the "Allergy Antidotes" system by Sandi Radomski
An exhaustive food elimination diet is well warranted!
Opioid like chemicals in dairy and wheat can cause psychiatric problems.
Increased immune sensitivity to gluten has also been reported in schizophrenic patients, and the connection between gluten intolerance and schizophrenia has been established for many years!
50% of people sensitive to gluten are also sensitive to dairy due to a process called cross-reactivity, where the immune system reacts the same to dairy as in gluten, since their molecular structure is very similar.
I think you should spend more time with your child in developing his interests more and more in activities like music, art, sports or books, to keep his mind occupied into something good and healthy, something positive, to help him keep his mind from wandering into thoughts that are negative or destructive toward self or others.
There are support groups, mostly in the UK and Australia, for people who hear voices. They have come up with numerous effective ways of dealing with these voices. (e.g. write a letter to the voice in my head. Negotiate that you will listen from 7:30 - 8:00pm if the voice(s) will be quiet the rest of the day so you can work. Learn that the voices don't have future predictive powers. etc.)