Anger in children can have many causes. First look at his environment, is it cluttered? Is it overstimulating? Keeping things simple and tidy can help reduce stress and anger in both children and adults.
Next look at his responsibilities, does he have any? Does he need to look to an adult for help with tasks, or is he allowed and able to do them on his own? Often times adults will do the cooking, cleaning, shopping, dressing, and planning for children. It may leave them feeling helpless, like an invalid, and purposeless. Making sure they have meaningful responsibilities that actually contribute to their day to day living can help reduce stress and anger.
Another aspect to look at is the structure of his days. Depending on the age of the child they may need more or less structure. Under the age of six they need a lot of structure and a lot of order. Frequent changes to their schedules can cause frustration and anger. Over that age too much structure can make them feel as if they have no say in their own lives. Also frustrating.
Finally when they having an angry outburst you need to teach them how to cope with their emotions. Kids need to learn a) what they are feeling, and b)how to deal with the feeling. I like to ask kids to take a deep breath, leave the room for a moment, or count to ten. Then I will wait and see how they are looking. You can usually tell if they are capable of talking or not by this point. If they are too angry to try calming down then just leave them alone for a minute, and then return. When they have calmed down then get down to their eye level and talk with them.
You will want to touch upon what made them angry, what they did when they were angry, how they could deal with that anger, and what they would/could do next time. Remember you need to ask what they think, not tell them what you think. Often anger is caused by feeling like their words or actions have no impact on the giants surrounding them.
One thing you do not want to do is cuddle, bribe, distract or reward angry outbursts. You want the child to learn to deal with their feelings, not reward them for dealing with them in a negative manner.
Hope this helps :)