My 11 year old nephew is so angry all the time. At times he can be so sweet, but most of the time he is moody and angry. He often tells his mother he hates her and most of his anger is directed toward her. Also, he still wets the bed. He is bright child but recently failed 3 classes and getting into fights in school.

Can you offer any advice on how to help him with anger management?

He needs to get into counseling. If he remains this way it will only get worse and then in a couple of years the parents will have NO control over him.
Something has to be troubling him. If he is open and communicates with you maybe you could have a heart to heart talk with him. I am thinking you may have a good relationship since you say how to help him with anger management?
Basically get him to realize he needs to stop and take a breath, calm down and rationalize the issues and the options of dealing with the problem in a more positive way and using a positive approach. Get him to realize that anger doesn't accomplish anything it often only creates more negative issues in the long run.
Maybe think of some examples and outcomes both negative and positive that he can relate to with his anger. Get him thinking through problem solving and conflict resolution.

Here are some things you can read through that might help

dealing with anger - Google Search (http://www.google.com/search?ie=UTF-8&oe=UTF-8&sourceid=navclient&gfns=1&q=dealing+with+anger)

coping with anger - Google Search (http://www.google.com/search?hl=en&q=coping+with+anger&btnG=Search)

anger management - Google Search (http://www.google.com/search?hl=en&q=anger+management+&btnG=Search)

problem solving and conflict resolution with anger - Google Search (http://www.google.com/search?hl=en&q=problem+solving+and+conflict+resolution+with+ang er+&btnG=Search)

My 12 year old nephew is the same way. Only, he gets violent. He has stabbed teachers with forks and hit them with chairs. He had a very traumatic experience a few years back and it has affected him deeply. He's in counseling and on meds. Has your nephew been through anything traumatic recently? That can trigger things like this.

Has your nephew been through anything traumatic recently? that can trigger things like this.

Even if we do not think they have been through anything traumatic it is amazing how kids can perceive things that we take as a necessary occurrence, routine or normal.
My son was violent for years and it all stemmed from his dad never being there for him.
He never learned to cope and now he basically hates his dad and doesn't want anything to do with him.