Horses are herd animals with a strong “peeking order, in which each horse or human has a place. For this horse you rank below him, and that means he feels no need to follow your instructions. Does he do this with other riders, or just you? If just you, then you need to figure out what the other riders are doing to make it clear that they rank higher than he does. If other riders have the same problem, this is just a lazy horse who has figured out how to get out of working and obeying any rider. He may be retrainable, but probably not by you. And long term, it might be less expensive and less frustrating and much much safer to find a different horse who is willing to accept you as his “head horse”.