What is wrong with my 6.5 y/o son?

I am at my wits end with my son some background info about him
-was checked for aspergers, autism, add, adhd, learning disorders....have determined none of these fit by professionals
-has had trouble sleeping pretty much since birth, is constantly tired and wakes up several times each night
-has been in speech and occupational therapy, which helped in those areas
-had a few behaviour problems in kindergarden, but nothing extreme and only lasted about a month

So the current issue is with school. Over the last couple of months his behaviour has gotten increasingly worse. it started out with general not listening and has escalated to violent outbursts. he has punched other kids in the face and tried to hit his teacher and principle. just today when asked why he threw something at the principal, he said because he wanted him dead. they have implimented a sticker system at school where they have broken the day down into 9 blocks, if he behaves for 3 blocks he gets extra time at the computer. i also reward/punish him at home on bad days he gets his DS taken away, on good days he gets to pick whatever he wants for supper that night. this has not been working for more than a few days in a row. i should also mention at home he is happy, awesome kid, never violent, never talks back. anyone know what to do from here because I have done everything suggested by teachers and doctors and it just seems to be getting worse.