He's a young hormonal chicken teenager, so don't take his behavior personally. He's working through what he's programmed to do and needs to figure out how to reconcile it with you and his flock. What you need to do is understand what that programming is; protect the flock and be the boss.
So don't be a threat. Striding right up to him, leaning over him, trying to grab him/handle him, or otherwise startling him (or any of the girls) is going to mark you as a threat. It doesn't matter how you raised him up until this point because if at any time you come off as a threat, he will respond as he is meant to (aggressively). You need to keep yourself off of his radar screen and convince him that you are what you are, a weird 2-legged feed dispenser. Not a bringer of death and doom. Keeping some treats with you at all times and always calling and offering them to him first whenever you first approach/come into view of the flock, is a great way to begin to solidify in his chicken brain that you're not going to hurt him or any of his flock.
Don't be a rival. You are NOT a chicken, so don't act like one. My DH and I screwed this part up with our roo at first. Whenever he'd try to have a go at us, we'd fight back (e.g. push him away or slap his beak). BIG no no, because it escalated his attack every single time. We were doing what he expected and giving him what he wanted. DON'T. It'll only encourage him to keep challenging you (because even if you "win", he'll try again...it's in his nature to always try to be alpha).
Instead, freeze and start talking to him as you normally would, in a calm voice (don't yell or scream or raise your voice). NOT moving would confuse the heck out of ours. He'd maintain battle posture and just stare and stare and lean from side to side and stare....at our shoes. He wouldn't flog if we didn't move and he'd only occasionally peck, so we were sure to wear sturdy pants and shoes through this training phase, but after a few seconds he'd snap out of it and walk away. Doing this did two things: 1) It allowed us to determine what triggered his attacks, and 2) diffused the situation without any battle or subjugating. What we learned was that our footwear and the how we approached him were the problem (and some research into his background with a previous owner clarified why it was what it was), and then we simply stopped doing those things (which were very simple and didn't cramp our style or activities around the flock a bit). Calling him and offering snacks as soon as he snapped out of battle mode and started walking away seemed to help a lot, too. He got rewarded for giving up and leaving and started reaching that point much quicker....and going into battle mode more and more rarely.
It took 3 months for us to work through all of this (he was 5 months old when we started the process), and this is where we are with him right now; when he first sees my DH or I he comes running like a prancing fool, calling the girls the whole time, and rushes right up to our knees and stand there and begs for snacks. Once he gets enough goodies to have distributed among the flock and saved a few for himself, they all go back to foraging and meandering about the property and we can go do/work on whatever we want anywhere on our property in close proximity to them or not. No one cares. He doesn't bother us and we don't bother him. Whenever they come up to us, we'll offer some scratches on the head or chest (even to him) and that's fine. We can pick the girls up if we want to. No problem there. He doesn't appreciate being picked up (and is tough to catch), but won't attack or fight you about it and is fine and easy to check for injuries/pests once you have him.
So we have our truce. However, it only extends to my DH and I. We're the ones who did all of the work with him, after all. So if you go down this road, make sure anyone who will be working with them often is also involved. Ours will go into pyscho kill mode on our neighbors/friends/family at the drop of a hat, so he must be in the run when a stranger is on the property. However, we're perfectly fine with that. It ensures a decided lack of unknown trespassers.