Don't give up yet. Young may be the keyword here. Young roosters tend to go through a hormonal phase, for a time, then as their hormones balance out, they settle down. This does not mean you can let him get away with his behavior. Since the change is recent, it's easier to try to get him to change. Assuming you are free ranging him, here are some things, from experience, that I suggest you try. For now, get a "rooster stick". A rooster stick can be anything from a cane, to a child's plastic bat, to a broom handle. For now, each time you go out in the yard, walk straight up to him. Of course, he should move out of your way, BUT there are a couple things to watch for when he stops. Watch his face. Is he giving you the stink eye? Keep him moving. Use the rooster stick for range, so you don't have to chase him as much, and you can tap him on the butt, if need be. As long as he's giving you the stink eye, keep him moving. Next, look at his stance when he comes to a stop. Is he relaxed, or is he doing the shoulder lean? If he's not relaxed, get him moving again. This can take a bit of time the first time, and you will have to do this for a couple weeks, but it should get easier. When you go out, and he moves out of your way, no stink eye, no shoulder lean, you don't have to keep him moving.
Ok, he's attacking from behind. You are prepared. The first time you get him moving, and keep him moving out of your way, until there is no stink eye, or shoulder lean, turn away to leave. Take a step, then turn around, and get him moving out of your way again. Turn away, take a couple steps, turn back around, and get him moving out of your way. Now turn around to walk away. If he tries to come after you, be ready to turn around, and swat him firmly with your rooster stick, and keep him moving out of your way. The object is to swat him firm enough he understands, but not enough to injure him. I don't suggest a garden tool for this, because it could injure him, and that's not what you are trying to do. Usually within a couple weeks of this, you can walk safely in your yard. Not always, but usually. I find that when it's a young rooster, and his hormones, a couple weeks of this solves the problem, however, always check for stink eye, and posturing in case you need to reinforce your rooster training at a later time.