Next time, pick him up and carry him around for awhile (the longer the better--at least 5-10 minutes; 30 is better, but usually not doable), holding him firmly more or less on his back and not letting him squirm and move abou. Make sure he is acting peaceful when you turn him loose. You are showing him that he can't affect you any more than he can affect a tree or a thunderstorm. It might take a few times, but it works. When you answer his challenge the way you are, you teach him that you are either a threat to his flock or a competitor for his hens, and that he can chase you away. When you act as a force of nature, challenging you gains him nothing.