A couple questions for ya:
1) What are you feeding him? Are you feeding him a lot of grain and if so what kind?
2) When you say he is jumpy, do you mean he spooks a lot or moves around, is hyper, what?
3) Is he this way on the ground?
4) Do you lunge before you ride?
5) Did you say he is an Arabian? If so (and I DO NOT mean this negatively) know that Arabs are very alert animals and are generally more high strung than some other breeds. Their stamina is incredible and their brains just go and go and go. They think a lot. LOL Simply put, his breed could be a contributing factor to this.
6) Are YOU nervous around him because you are anticipating his behavior? You may not think you are, but chances are, you very well might be sending him a signal that you are nervous, therefore, because of #5 he is going to get nervous, jumpy, whatever.
I'll wait for your reply and see if I can come up with some advice or techniques for you. :)
"No hour of life is wasted that is spent in the saddle." --Winston Churchill