G&S’s suggestions are the best. Do you know your horse’s background? I have two Arabians – one who won’t even consider jumping (abusive past owners whose kids just wanted to hit and jump), and one who will, when turned out in the ring, go around jumping everything because he likes it way too much. Sometimes, if there is a bad association with something, it will take a really long time and a lot of patience to get over it. And, not all horses want to jump, even though they can. If this is his only quirk, you are very lucky!
It is never the horse's fault