Personally, I think he’s being a brat, senses your fear, and is taking full advantage of it. It most likely is something he picked up being a lesson horse over the Summer, and now you’re stuck with the problems. Your anxiety is most likely making it worse, but I don’t suggest riding your horse through a kicking episode because one of you is going to get hurt. My suggestion? Go back to basics. I would start from the ground, and then move up from there. On the ground, go over basic suppling exercises and teach him to give to pressure on both sides. Pull the rein gently toward his shoulder, and when he gives, reward him by instantly releasing the rein. This will help you control him if he decides to be foolish in the arena while cantering. As for the group cantering issue, I would lunge him while the other horses are doing their thing, and insist he focus on you. When they start to canter, insist he just walk. Ask him to canter while they are doing something else, just make sure he’s not looking around getting distracted. The point here is to keep his focus on you. Then canter him while they are cantering. When he seems to calm down, then you can take the saddle and hopefully his problems will have ironed themselves out. I hope I was able to help you. If you have any other questions, don’t hesitate to ask. Happy riding!!