You both need to relax, and it may be harder for you than for him (I struggle with this issue myself). He is still very young, and may just want to leap for fun at times, but this is a very bad thing if you are sitting on him. Horses know if you are nervous, and since you are the herd leader, they assume that there is danger somewhere, which makes them worse. That then makes you more nervous, and so it goes on and on. Is there someone you trust who could work with you both?
It is never the horse's fault