I know what you are going through. I am afraid that if you do not deal with the problems in school now they will escalate. Put in place a consistant punishment/reward program for when he is good or bad in school each day. Make sure you talk to his teacher on a daily basis, maybe after school, that is when I talk to my sons teacher. Also, do this for home as well, and like the doctor said, seek help, they can help you too in regards to dealing with the problems.
First, you should arrange an evaluation with a pediatric mental health clincian. The behavior in school is far outside the normal range. At home, take a look at Lynn Clark's book SOS Help for Parents and follow its guidelines carefully (see www.sosprograms.com).