I'm gonna have to agree with MIEventer on this one. He could be anxious from previous training, could be a lack of desire, could be you, could be his environment, could also be pain. You might want to get him check out (and his tack too) and make sure jumping isn't hurting him somehow.
first of all, how are his basics? Is he solid at the w/t/c? Can he slow or speed up when asked? does he move off your leg? is he balanced? can you rebalance if he gets in a situation where he isn't? he must be able to do all these things well before jumping!
If he can't trot over poles confidently he sure as heck isn't going to be confident over fences! I'd take it down a notch and work on poles for a while. lots and lots of poles! trotting and cantering. single poles, series of poles, etc. For trot poles they should be spaced 4 feet apart for the average horse (who naturally has a 12 foot stride at the canter) and 9 feet apart for a series of canter poles. I'd start off with 1 pole at a time, over and over until he's bored. You don't have to approach the pole perpendicularly, you can angle your approach, but keep your path fairly straight (meaning don't turn over the pole). Then move on to a series of poles. An exercise i did recently was to put 8 trot poles out! It was awesome! REALLY made the horses think and figure out where to put their legs. You should start easy though with just 3 trot poles. That's just hard enough that they have to think and easy enough that they can get through it even if they mess up. Another good one is 3 canter poles (all 9 feet apart).
When he is confident over poles move him on to a small crossbar and treat it just like you would a single pole. If he tries to stop KEEP HIM GOING! Your horse can step over it, do NOT let him pause! Do NOT let him sniff it! (if he must sniff it, do it before your approach to it) MAKE him go over it and reward him on the other side. If he just trots over it pick up a canter as soon as you 'land', that will encourage him to jump it and land at a canter. Do it only at a trot (for a long long time) unless you are very very confident at the canter and have a very accurate eye for a distance.
I have no idea what you look like when you're riding or what your skills are but I'm willing to guess that you are getting anxious or apprehensive to the fence and anticipating the stop. Your body language and form are EXTREMELY important when it comes to jumping a green horse! Most scared horses will start asking "are you sure" long before they get to the fence. Close your leg on and reinforce that you do want to go over it! Keep your eyes up (OHHHHH soo important) and have the expectation that your horse is going over. stopping at fences is self fulfilling prophecy. If you think he's going to stop? most likely they will. Careful you don't throw yourself over the fence, do not get tense, do NOT lean forward infront of the fence in anticipation. You must be very very still with your body but firm that you guys are going forward, and he is going over. I took a clinic with someone on a terrible stopper who told me "throw your heart over the fence and catch it on the other side". She meant, KNOW you're going over the fence and go for it! You can't be wishy washy or fearful when you ride a scared horse. Remember that you are his herd leader. He is looking to you for direction. If you are afraid (because he's going to stop) he is going to assume you too are afraid of the fence. So you can't anticipate.
You are absolutely correct that you can't even think about doing a course yet. That would fry the brain of many scared jumpers! Once he can do poles confidently move on the crossbar singles. However, what you really want to train early on is the concept of "keep going". Every time your horse lands from a jump you want him to be looking for his next fence. So after you do your single (unless he's a complete nut) I'd start doing some trot courses with lots of space inbetween the fences. So you'd trot over a fence, land at a canter, go back to a trot and do something like a figure 8 or some serpentines, then go over another fence, trot and do another short exercise, go over another fence, etc. Don't just go over the fence and halt straight and then walk around on a loose rein. (unless he really needs time to 'think' about what just happened, some do) When he's confident at that then try cantering a few fences and moving on to lines (trotting in cantering out, long if you can. like 5 or 6 strides). I usually do some easy gymnastics before I do lines, because it helps them get the concept of cantering into a line so much easier, but taht's just me.
There are some people out there who teach their horse over singles and then up the height to 2"3 or so (or higher
) and then teach courses. I think it's so much easier for them to teach them a course at 18 in and then raise the fences up. I don't know if there's a right answer. But it seems to me that once a horse can jump a course confidently at 18 in moving the fences up higher is nothing to them.
anyways. i've written a novel as i so often do.
i hope it helps at least a little bit! keep us updated!