If I recall correctly, that's the age when boys think it's hilarious to do things like pretend to slam their faces into doors and so forth. Whether he intentionally caused you to hit him or not, he was probably trying to scare you and misjudged the situation. Even if his intentions were good and he got confused, I'd say he was old enough to know to stay in one place as you went by. You definitely did everything you could to warn him of your presence and intention, and if he chooses to channel chipmunk behavior instead of acting like a moderately intelligent human being, that's his decision.