Horse is over jumping?

Have him trot and jump it for a bit and don't give him so much room to trot so instead of a good running start only about 10-15 ft of a trotting start. Maybe before you started using him he was used for a high jumper or he had a bad accident jumping and now he is worried about not clearing the jump. and if he is a quarter horse they have strong hind quarters so it would be natural