He is challanging you. It's time to show him you are the top rooster and no matter what he does it is not going to change.
Here is what I do.
1. Too late for this, but from the very beginning as chicks I always pick up, hold and carry around the roosters I plan on keeping. This generally shows them that I am much stronger and smarter then them and they usually don't even try to challange you. I have three roosters now and I can walk in and pick any of them up. They don't even try to get away, they just let me do it.
2. Once the rooster attacks you immediately need to turn around and go after him. Chase him down and give a soccer type lift kick. Before folks jump on me here is what I mean. Take you foot and get it under the roo and lift up and give him a helpful lift to the other side of the coop or run. Roosters when they fight do a lot of fighting with there feet and legs trying to knock each other off balance, this freee ride from on side of the run to the other shows him you are definately the stronger rooster. Once you do this go after him again and catch him pick him up and carry him around for 5 or 10 minutes.
Then for the next week go in and before you do anything else go after the rooster, catch him, hold him and carry him around a bit.
This method has generally worked for me. I did have on RIR rooster years ago that was bound and determined to whoop me and I could not break him of flogging me when my back was turned. (big chicken!!)
3. If the above fail, it's chicken and dumplings for dinner.