Why is my horse such a butt when it comes to flatwork?

He could be bored. He knows that jumping is much more exciting than walking circles over and over again. He is also learning that when he misbehaves, you put up a jump (in an attempt to get him to cooperate) and he gets to have some fun. This is just reinforcing his behavior-he bucks, you let him jump. You should work on ground manners with him. Work with him on the ground and do things like weaving through a line of cones or jump standards, walking around barrels, and other things. He really needs to learn some manners and understand that he cannot dictate what to do when you ride and that you are the boss. If you feel safe enough, maybe take him on a trail ride or ride around an empty pasture. Also when working with him, have an experienced horse-person with you to watch him and help you out. Good luck!