You may want to do some groundwork. He might need to re-learn or learn to give to pressure. From the ground, start pushing him with a finger or the end of a crop. Increase the pressure until he moves away. Once he moves immediately remove the pressure and praise him. Do this from all four corners and the middle. Do it a lot, then transfer that lesson to the saddle with your leg giving the pressure to move one part of his body or the other.
Sorry if you already know this.
"Until one has loved an animal, part of one's soul remains unawakened..."
- Anatole France