When he displays the aggression, grab him by the feathers on the back of his neck, and hold his head down to the ground, until he ceases to struggle. Do this every time. Or you can peck him back with your finger. Every time you correct him, say NO! Or you could have a box handy, and when he displays aggression to you, plop him in the box until you're finished doing what you came to do. Take your time finishing your chicken chores, while you're at it! They can be trained. But, some birds are beyond training. they make wonderful soup. If training doesn't work, you may need to remove him before he is soup worthy. My roo knows the meaning of the word "NO!" I'm convinced he knows his name, also! When he's chasing down a pullet, I verbally reprimand him, and he will stop, give me the "You talking to me?" look, and go find something else to do.