How is his home? A child lives a lot of what he sees,  if it worked once, it'll work again. They are so sharp, they will try as many times they can. Example is a lot that inspires a child start by complimenting somebody that you like their behavior in front of your child, we all look for attention and do more of what you want to see.

You should make it funny and show the way he should act instead. I am working on this with my 3 year old and have had a little change already. When he steps on something and hurts his foot make a funny face and say oww with him. Hope it goes well! :)

Hi there,
It is very challenging and one needs to be creative in dealing with this negative emotion.  You could try the following:
1. To talk to him about what can happen when there is negative channeling the anger.
2. Ask him how he feels and how he can help himself to channel the anger.
3. Set up an anger management task at home and at school so that he can work to eliminating the anger.  The carry over from school to home and vice versa is important.
4.  Ask the teacher to have a punching bag in the class that he can go to when he is feeling angry.
5.  Create a time out corner that he goes to when he has the urge to lash out.
6.  Always make him aware that you still love him but not the actions.
7. Encourage him to do indoor/outdoor gardening projects.
Consequences can be taking away things he likes or doing chores that he does not like to do.
Hope you find them helpful and good luck.

Martial arts will be a great fit for you and your son!  A good taekwondo school, for example, will have instructors ready to assist you in dealing with problems at school.  These instructors are ready to advise you and your child on the (personally and legally) appropriate methods of handing situations at school.  The discipline in martial arts lends itself toward pulling out the problems and putting them at the front of the child's development.

As he is still young, he may be having trouble communicating some source issue that is spurring the outburst.  My son got into a fight with another child in Kindergarten, completely outside his nature, and we discovered two days later that he had strep throat and had probably not felt well the day of the fight.  He, of course, could not relate this to us, but sometimes we parents need these experiences to uncover the signs with our children.  If the outbursts happen during class, show up during class and observe them.  If they happen during recess, make some surprise visits and observe him.  If he is responding to the provocation of other children, or to some miscommunication with a teacher that he cannot verbalize to you, you may be able to piece the puzzle together in this manner.