I have the same problem with my younger brother. First of all is he much smaller than you? My brother is and you have to look at it from their point of view. Also does he like boxing? My brother doesn't really like boxing and he doesn't want to hit me. You can't make someone like anything. Also remember sparring is for learning purposes and not to hurt someone. It is not a competition so take it easy and try to take something away with you from each sparring session.