I'm working with my son on anger management when he is being disciplined. Is it okay to reward him with things like picking a family movie when he is successful at expressing his anger/frustration in an acceptable way?
I don't recommend it. Rewarding a child for demonstrating self-control can actually exacerbate the problem. The child may intuitively realize that he only receives a reward because he usually does NOT control his anger. So he lets fly with nine outbursts of anger when disciplined in order to be rewarded with going to the movies with a friend when, on the tenth occasion, he controls himself.
Quite often, it teaches them how to manipulate other people. When your son demonstrates good self-control, you may or may not see fit to tell him later that you were pleased. But I would not make a big deal of behavior that is expected.