How can I train my horse to move off my leg?

If your horse moves away from leg pressure on the ground, he should move away under saddle. However, if your horse is tall and you are very short legged, you may be cueing him too high on his rib cage for him to understand. If not, maybe you need some very small blunt English spurs to give him the message. As for being herd bound, that's a common problem in insecure or inexperienced horses. Try hauling him away from his stablemates to an unfamiliar area or riding him with someone on an unfamiliar horse.