I don't believe that an aggressive rooster can be trained. Aggression is an inherited trait, and I don't believe an aggressive roo should not be allowed to breed, either.
You need to treat the rooster like a rooster, not like a person to be reasoned with. Roosters don't understand "treat gently" any more than an attacking dog does. Yes, you can make friends with an attacking dog, but that dog will never be "safe" and neither will your rooster.
If you really want to keep this rooster, you can try acting like the bigger rooster. If you're attacked, boot him, squawk, and chase that rooster around the yard while you squawk. Catch him and pin him to the ground. Pick him up and carry him upside down often and show him that YOU are the flock boss. The problem with this method is that a junior rooster is always angling to be the boss rooster, so that rooster might come back at you a day, a week, a month, or a year later, or never. You'll never know.
Roosters are dangerous proportional to their size, but they are dangerous for the same reasons as a bull or a stallion, and unpredictable in the same way, too. You must treat them with the respect they deserve as flock master and be wary of them, while at the same time maintaining your top spot in the flock. And certainly never let any children anywhere near the animal.
I'm not advocating abuse on a rooster just to abuse. I've only ever booted my head rooster ONCE, when he was young and feisty and thought maybe he's see what I was made of. And he didn't flog me or peck me, he just came up behind me with his neck feathers fluffed out and his head low. I let him have it, and must have looked like an idiot doing it--chasing him around the pasture squawking and hollering at him. He's never challenged me since and is a very nice flock leader. I do remind him that he's mine by picking him up occasionally, turning him upside down, messing with his feathers, etc and not letting him go until he's stopped fighting me. I've never had to do anything to my subordinate roos, since they've never tried anything.
I say that best thing to do with attacking roosters is chicken noodle soup. Roosters are easy to come by, and there are many looking for homes every day. Most are free, even pure bred ones.