Help with my horse please?

I have a warm blood/ Thoroughbred cross, and im having issues with him with that jumps. (im a hunter btw) Its not going over the jump i have problems with, its in the corners, his approach and how he moves away from the jump. He has a veryyy big head, with bulky shoulders so sometimes i have issues keeping him straight and packaged. He often drops his inside shoulder and leans. after the jumps he tries to dive in. Also, he used to have a perfect lead change, but starting this year he only swaps his front legs and cross canters until i make him do the simple. My trainers thinks it might have to do with an old torn tendon injury that could be irritated. any advice or other observations?