Wow! He's running so fast that he's not stopping!
Oh no! Why did he stop?!
Well, you see, Newton's First Law states that every object in a state of uniform motion will remain in that state of motion unless an external force acts upon it. He was running at a uniform rate until he ran into the wall.
Newton also says that an object at rest will stay at rest unless an external force acts on it. So, by that logic, he'll stay there now until an external force acts on him.