Wow! Well, first of all it would be a great idea to try and find out the reason for the sudden change. There is something that triggered this. Something that you could possibly try in addition to discipline is positive reinforcement. For instance, make a chart with your son of things that he does around the house or at school. Each time that he does it, put a check. At the bottom of the chart have various rewards/activities that he can get if he reaches a certain number of checks. For instance, he ate his dinner, he completed his homework, he received a positive comment from his teacher, he went to sleep without crying...then he could receive a check for each category. If you let him choose some possible rewards, he will work harder to reach a goal. Chuck E Cheese, go to McDonalds, go skating for instance can be some choices. 

Each time he gets a positive note or comment from his teacher, I would make it seem like it is the best thing ever...like he just became a millionaire. He will love to please you and make you happy with him.

 

I hope this helps. Good luck!