I've got a son who has Asperger's disorder and ADHD. He can be very violent.

Something you can try is to use pen and paper and draw to him. Draw with him after something has happen but when he's calmed down. Draw an image of himself and the person he's been hitting and tears rolling down that persons face... draw to let him know where it hurts. Another thing is to teach him to read faces... draw simple faces... smiling, crying, being angry and so on and tell him the meaning of these faces. You can also try for him to point out which face he identify himself with at that moment... all after what you think he can take in at his young age.
It's hard work to do this over and over again... but often it shows result.

Something very important when drawing... is to ever time draw one final image showing the right behaviour and smiling faces telling it's something really good.
Please let me know how it goes...

Good luck! I feel for you in this!