9 year old with agression problems

Our son is 9 and he has been having problems with anger since he was 6. Most of his anger is at school. It started the end of 1st grade, when he was being defiant to the teacher. Then in 2nd grade he got in trouble for threating a kid and the for hitting the teachers and Principal. He was then kicked out of that school and placed in another. It was still happening. Then he went to physical abuse to kids and staff at the school. Summer was good that year. 3rd grade was worse yet. He hit a lot of kids at school and then one day I had to pick him up from the police station because he kicked a 4th grade girl down the bus steps, hit and cursed at the bus driver and the Principal. He then took of running from the bus to a main street where the Principal caught him and brought him back to school kicking screaming and cursing. The police were called by the school and they took my son to the police station where I picked him up. Again he was kicked out of that school and placed into a Special Education class. He was evaluated and they found nothing wrong. They advised me to also take him to a psychologist. I did and that last for 6 months, but no progress and they couldn't find anything wrong. So now (3rd grade Special Education class) he was doing great the last 3 1/2 months of third grade was great even all the way through summer. Then we get stuff from the school saying they lost the budget for the busing of students out of the school district so my son was going to have to go to a different school for 4th. Well 4th grade just started and we are not even a month in and he has been in trouble already two days in a row. I have read lots of stuff on how to  discipline   nothing works. I have tried reward systems, time outs, grounding, taking stuff away and making him earn it back, sitting down talking and not punishing and nothing works. Please help. I just know if he is like this now he will get worse as he get older. Keep in mind he is a big boy, he is 9 years old 4'6 and weighs 102lbs, which makes him scary to other kids, but he is also super shy. He is a follower and not a leader. He has a good heart but we can't get him to stop. Again please help