Oh please don't punish. Punishment teaches to punish. The child feels bad, self-esteem goes down and the result is either more aggression, or, in the long term depressin, adictions OCD... fear is not a good teacher. It teaches that mom is not safe, and that something is wrong with who the child is. He is trying so hard to be the right child for you.

Where did I learn this? From the book, Raising Our Children, Raising Ourselves. Do yourself a favor and read the book. Listen to your child. Try to understand the reason behind his loss of control so you can take care of him and prevent him from needing to do it. Only love is a teacher. Read that amazing book and learn how to have well behave child who behave so well not because of fear of funishment or disproval but because he really wants to.

What do the Kindergarten teacher's say? How are they dealing with it? Yes, you must re-enforce that hitting is not good.. but if he's been told off or punished at school, does he need to be punished again?