I agree with the above post. Roosters are a crap shoot, and they are always the darling as chicks. "People get attached to them, and just wish they would be like that forever." AND that is the problem.
They might stay nice, and they might not, it is a crap shoot. Decide and have a plan what to do if they don't stay nice. That is being responsible. It has to be more of a plan than just wishing they would be nice. My plan is to have a way to catch him, (a 5 foot handled fish net works well) and a place to confine him until I can do something else with him.
I know you are hoping that training can help, but if you read on here, I have read a lot of training advice, completely opposite of each other, when I came to the conclusion, people think that this worked or that worked, but really they just got lucky.
Mrs K