Most of the time it's NOT you. This is the boys version of a period, they like to keep everything inside. However, if you've tried talking to him face to face and he just ignores you then you may want to ask one of his friends to talk to him about it. But if all else fails force him to sit and talk with you. Don't jerk him but just go up to him and ask him why he's been ignoring you. Confront him.