R.S.
Make sure that there are no underlying issues at school -- how does the teacher behave toward him? Often boys, especially if they are rambunctious, get unfairly labelled as "bad" early and are treated accordingly, getting punished more frequently and more harshly than other children. What about the rest of the staff (principal, etc.) Are they interested in finding out more about what is going on? Any other caretakers in the school that you may not be aware about? I have had many experiences in which I will pick up my kid from day care to find him under the care of a person I have never met before. What are they doing with your kids? Any class bullies? A little boy or girl who is teasing him? Anyone acting inappropriately with your child (being mean, hitting, even inappropriate touching/hugging)?
Especially if he is well behaved at home, I would take a very, very close look at this school. Perhaps taking a day or two to spend at the school and observe the environment may make a big difference. Maybe drop by at an unexpected time, to really catch the staff at work when no parents are around.
Finally, I would try to talk to some of the other parents in the class. Anyone else having similar problems with their kids? if so, maybe you can team up with another parent to investigate the school together, and watch out for each other's children if necessary.