He might be feeling you get a bit apprehensive about the jump. If you tense up at all he may think he should "worry" about it and just freeze. Also have you jumped before/taken lessons in jumping. Riding english on the flat is different than jumping. Please get the help of an experienced trainer to help train your horse and to help you learn the proper fundementals of jumping. It's not just ride up to the jump and fling the horse over it and hope for the best.
I would start him on the ground. It's a lot easier for a horse to find their balance over a jump the first good amount of times without a rider than with. IMHO.
If you have a round pen I would set up 2 poles on the ground on either side of the round pen across from each other. Send him around at each gait in both directions a few times.
Once he seems comfortable and relaxed at each gait and in each direction (not rushing, panicing, balking, etc) set up "standards" on the sides of each pole but don't raise the poles up. Then once again send him around at all gaits and in both directions.
Once he's relaxed with that raise one side of the poles about 6 inches off the ground. Never place the poles in a fence where it can get caught up if the horse hits it. It could be disasterous.
Continue through the process of raising the poles up(both sides) and add in a small cross rail(after a while of working with real small "jumps") I would stay at cross rails for quite some time before moving up to a straight vertical jump. From what I've found cross rails seem to give the horse a sense of security( I don't know why but every horse I've worked with would happily hop over a cross rail and then shy away at a 1 foot vertical) also while riding cross rails help you to guide your horse to the center of the jump. When you do move up to verticals(after quite some time) add a ground pole at the bottom of the jump to give the horse a guideline.
recommend trying to move up the height a lot during one session. Work at it on the ground consistantly for a few weeks or so and then go to the saddle.
When you go to the saddle start back at the ground pole and work your way up slowly. Make sure you're not catching the horse in the mouth on the landing (when you move up that high which shouldn't be for a while)
Some horses you can just take and point at a 2ft jump and they'll take it even if they know how to or not. Others need time to understand and figure out what you're asking them. So instead of just willy nilly bound over the jump like in the movies take the time to teach your horse to understand what you want.
It's also not something you can do overnight. Sure you could "force" the horse over the jump (not saying you are, just an example) but what would you be teaching the horse? At the end of every lesson the horse should be calmer(NOT tired or exhausted) than when you started IMO.
I'll say it again please get the help of an experienced trainer because it's a real trainwreck trying to teach yourself to jump while trying to teach your horse.