Good question!  At this age children's behavior can be very impulsive and immediate with no thought of consequence.  It may be helpful to lay out some very clear rules for this youngster and then follow them precisely.  

First, write out what the rules are very clearly.  With children of this age, pictures may be the most effective.  Then help explain why these are the rules.  Explain how hitting is harmful to his classmates and the environment at school.  After explaining the reason for having the rule, then write out what the consequences are, once again unsing pictures, explaining why that is the consequence.  And then as soon as the rules are broken, refer to what was written down and implement the consequence.  Hopefully then you can help the child begin to recognize that all of his actions have consequences.  Also have consequences for when he is well behaved.  It is good to extinguish bad behaviors, but perhaps even more important to cultivate good behaviors.  Catch him being good.  

If these rules are not working, it will be helpful to work with the other staff at the school to see if they have any other ideas.  Also talk with the student's parents and see if they have any ideas of how to help.  If he continues to be disruptive, it may be necessary to remove the student from the class altogether and find an alternative educational option until he is ready to partake in the learning environment in a healthy way.  If possible, it would also be helpful for the child to see a counselor or child psychologist so that his specific issues and problems can be directly addressed.  Good luck and all the best!