I wore an unbuttoned, old flannel shirt when I went out. Whenever he would get "that look," I would lift the flannel like wings, flap a few times, and make growling clucking sounds. Sometimes he wouldn't back down, and I would have to chase him a few steps. (Can only imagine what the neighbors thought! A chunky old lady, flannel "wings" raised, growling/clucking/crowing, chasing a little lilac roo around the backyard!) I would stop when his aggressive stance shifted. Then I would shake my wings back to normal and croon softly, ignoring him and returning to whatever I was doing. Most of the new flock stayed with me, and the 2 spazzy pullets who followed only him were rehomed with him. "Take this rooster and get 2 pullets free!" (Traded a dozen eggs for them back when eggs were outrageous!)