My dog is snapping at my baby brother?

I've have Harli (a 12 pound italian grey hound) for over a year now and have never had an issue with him snapping at anyone including my four year old little brother (even though my brother has always played rough with him). Then yesterday Harli had a bone he was chewing on and my brother tried to come up and pet him, Harli snapped at him and actually bit his nose hard enough to make him bleed. We thought it must have been isolated incident caused by the new bone but today they were playing together with a rope and Harli did it again getting alarmingly close to my brother's eye. My family has discussed it and decided to first keep the two separated and work on training techniques, but I need help on how to train him, Does anyone have any tips for establishing that my brother has higher dominance in the pack and/or stopping this behavior?