He didn't have any control of Hulk's actions in the middle of the movie. He lost control and then he wasn't able to take it back. But then in the climax, he was suddenly in control of the damage he was doing and even rescued Iron Man. Which meant Bruce Banner had total control as Hulk.
Did they explain how he managed to do that? In all his conversations before the climax battle, he was talking as if he had not taken control over Hulk. He was able to not bring it out but he didn't seem to have control of it. Then in final battle, he was in total control.
If I missed something, please let me know.