No, don't kneel down! You
don't want to be at eye level with him! He moves faster and has more sharp stuff than you do...
My experience has been that kicking just made them more aqgressive. I taught mine to back down by walking straight at them, and waggling bags or sticks in their faces until they backed up. I made them all stay 3 feet away from me. Just walk briskly at him like he's not there, and make him give way.
Also, note the preliminaries to aggression. That will be a funny sideways walk at you, or walking a little circle around you. That's an assertion of dominance. Back him up any time he starts that (or when he does it to a hen near you), so that you can be established as dominant
before it gets to a fight.
I had 26 roosters until a few weeks ago (now most of the roosters are dinner). The roosters weren't afraid of people, and would come up close but bite hard when they were petted. So I started training them - it was pretty cool to walk down to the coop with roosters parting before me like the Red Sea
I think it's important to start training them as soon as you can tell they're roosters,
before they really attack anybody.