Angry son

My son is 10. He is the middle child of 3. He is usually a happy boy....until he doesnt get his way. Which he has not always been like this just over the past year I have noticed him getting so mad at things (ex: not wanting to share a toy with his little sister and I make him) he blows up he starts breathing weird and sometimes balls up his fist,mummbles under his breath or yells at her and calls her a stupid baby! I have talked to him and told him that it is not right or nice. He needs to love his sister and treat her how he wants to be treated. I got a phone call today from his teacher....he is slacking off in class, drawing instesd of paying attention and on the playground he said to another child "I'm going to kill you b****" I could not believe that my son said that!
He also tests me alot... he moves like a turtle when I ask him to do something or gets an attitude with me. I have to go to his father to really get him to listen to me sometimes. I feel I should not have to do that. It breaks my heart to see him acting out like this.
Can you please give me some guidence on how to handle this...before it gets worse.

[margypops]

It is his toy perhaps allow him to decide when he wants to share it not tell him to when you want him to.I have seen this many times when the older child has to give up their special toy to placate a younger sibling.  He is feeling jealousy in regards to his little sister, are you unwittingly feeding this, focus on his positive side and praise him when you see him doing something right.The fact he says stupid baby tells you thye facts, so if the Baby yells for his toy tell her No , let him decide if and when he wants to share it.