From what I have read, this is very common once the cockeral reaches maturity. Even more so with a bird that you have handled a lot or have been friendly with up until his transformation. The friendliness you have established with him now turns into a need to dominate once his natural instincts kick in. It is more likely he will be aggressive with you if he is already not afraid of you. I am told if this happens, you need to establish dominance with him, just as would happen in the flock. When attacked, you need to subdue him.
Get a hold of him and simply hold him to the ground for a few minutes. Do this everytime he is aggressive and he will soon settle into his place of not top dog, but second in command. If the aggression does not abate, then you have other options. This is simply the social order of chickens. The rooster is not suddenly becoming mean or evil. This is chickens being chickens.