show your horse the jump from the ground. Bring him to the jump and pat him and give him a small treat. This will show him that the jump isn't such a big deal because when he goes to jump it he will remember that you have him a pat and he won't be so scared. I did this when my horse was scared and it worked really well :)
Walk the horse up the the jump and let him just see it and look at it for acouple minutes.
When you get the the jump add ALOT of leg. Normally horses tend to choose one side to duck out to(such as when my horse refuses he ducks out to the left) if you've noticed a pattern make sure you have more leg on that side. For example when I can feel my horse getting ready to duck out I add ALOT of left leg and sometimes I kick.
If that doesn't work then use a crop. Hit him on the shoulder and/or butt.
You need to MAKE him get over the jump because if you don't then he will ALWAYS take advantage of you and it may even become a bad habit and may even start to refuse other and all jumps.
If the crop doesn't work you can have what my trainer tried once.
Have someone stand on the ground next to the jump with a long whip and when the horse comes up to the jump either have the person on the ground slap the whip on the ground behind the horse OR they can tap the back leg of the horse.
The horse may also feel like you KNOW it's coming. I havnt seen you ride so I can't really say anything but are you confident over the jump? If you are any bit UNconfident your horse will sense it and that's another reason he will refuse it. Sit up, add leg and get him over the jump. Show him that your the boss, don't be mean about it but show him your boss and that you won't let him get away with anything, and do that by being confident!
It might help to figure out why this jump? Is there something different about it than the others? A funky filler? Bright colour? Is it into a shadow? Pointing away from the gate? Do you tense up approaching this jump?
I'd say let him sniff and have a good look at it so he's not afraid of it. Maybe lower it to begin with or have another horse lead you over