This is free lunging we are talking about? Or on line? If free lunging, do plenty of changes of direction if he starts to thinking about bucking. He can buck, but he will end up having to turn and go the other way. Don't get mad or upset, just move him on. Do NOT get emotional when he kicks out , but if he comes in to you, move him out immediately. The sooner you can cautch that behavior, the easier it is to change. If he starts to come in when you are asking for forward on the circle, don't wait unti he's close to you. He already made a decision counter to your request; he decided to come in. YOu interrupt that decision before he gets to act three steps on it. From then on, you watch his face and if you see him thinking about it, you give him more work to think about. I dont' mean you punish him, you just put your request for "forward" more obviously in the front of his mind. YOu remind him that you are there and you are a meaningful presence and he'd better be looking because he cannot assume what you are going to do, he needs to watch for the next direction, be it, turn , speed up, slow down, back up , come in to me, go out from me, stand and face me , etc.
The horse is "complaining" about the changes in his life. But, if you give him a lot to think about he will let go of that irritable resistance.