best advice I can give is to nip it in the bud now. Have your son stand up to him also. He has to be scared of you or he'll lord it over you.

If you're near him, "peck" him on his head and chase him off for no reason. If you see him mating, chase him off the girl. You can even try to imitate the "wing dance" to the girl right afterward. Then crow like you own the place (well, you do own the place!) Always walk into the coop with your head and shoulders held high like you're big and bad. Yes, it looks dumb to us, but chickens understand body language best. You can also handle him more. Pick him up and carry him around. It will embarrass him. Pick up his girls, too.