Is someone going out of their way to get on his nerves or to cause problems?
I think I understoood you to say that this is the same place he went to kindy?
As a child care center owner ... generally we don't see things that are "out of character". Parents tell us they don't act that way at home, but they don't have 12-20 kids to try and get along with. School is more structured and there are more rules. Child Care, EVEN summer Camp care, is more laid back and children being allowed the be children. Do they tell you what is happening before he starts acting out? Maybe he is play wrestling and it gets out of hand? Are his "teachers" preK teachers or are they school age teachers? If they are used to tending to pre-K or younger, they may not know what they are doing with the older kids and don't structure their day enough.
If you can, pop in and observe the teachers and what they are doing ... and what the other kids are doing ... as well as your son. Maybe this place just is not for him.
Try involving your son in a discussion about what is going on. "Wow, you were really mad today." ( Leave room for him to talk) "Sometime I get mad too, can I tell you what I do?" Give options, talking to an adult, expressing feelings calmly, walking away. Also see if it is physical. Do these altercations happen around the same time each day? Is he hungry? Tired? Ask yourselves if this is new? Have there been changes in your sons life? And lastly you are the most influential people in his life as his parents, how do you conduct yourselves when you are angry? Are you volatile, even with words or volume? Do you solve situations with violence, such as spanking? Answering some of these questions may lead to solutions.
I wish you the best.