Hi there. Oh, well . . . I'm sad for your boy. Often behavior issues are a symptom of inner turmoil. Could he be sad about a split you had with his father? That you are busy with a new boyfriend? Could he be stressed about anything?
If you remember that kids show how they feel on the inside with outside behaviors---- it will help you be less angry about his behavior and then be able to problem solve it.
For example, you are asking him to do something he doesn't want to do. What are you asking him to do? Is there a reason he doesn't want to? Is it hard for him. My son has trouble with handwriting. He is resistant to it because it is hard. So think about what triggers can be involved.
Also, is there any adult in his life that has a habit of speaking to others rudely or using angry tones?
A good book to read is "love and logic" by Charles Fey. It goes through a parenting system that is really helpful in my opinion (mother of a 7 year old and 8 year old--- both boys).
If his behavior continues to be an issue at school, you can ask to have him evaluated. In most states, they must do that upon parent requests. If his academics are affected, you can apply for an IEP but if it is only behavior, you will ask for a 504 plan. Many schools are reluctant to give any plan unless academic performance is below the standards set for kids at the grade level your child is in. You're your child's advocate, so cross that bridge when you come to it but advocates are very helpful in those matters.
Okay, good luck
Have you had him evaluated for a developmental disability? Since the school has noticed it I would call a meeting with them to discuss it. I would also talk to his pediatrician and get a referral to a developmental pediatrician. He may be in need of an IEP. He is struggling and it's dragging him down. Everyone will be so much happier if you get him some help. It is important to lay down the law and exact punishment when necessary, but it is also important to figure out what is causing the behavior and address that.