Get hold of a copy of Teach Yourself Horse by Heather Simpson. This will give you a great start at understanding horse behaviour or find your nearest qualified Natural Animal Centre equine graduate for a consultation.

If he's not in any pain from back/legs/teeth, etc., then my advice is to do lots of schooling, maybe even going back to basics to make sure he knows what you're asking and is happy with his work. If he hasn't jumped before, much or at all, then he just needs lots of practice and a very steady, consistent rider to help teach him. Make sure you have a confident, balanced position while jumping so he can get on with his jumping. You could try riding in the light seat so you're off his back completely. Do lots of polework so he has the muscles to cope with jumping, and to get him used to the poles themselves. Maybe ask a more experienced rider to watch you ride and have a go in case you're inadvertently doing something to put him off - they might be able to see signs of what is causing him to stop, which you won't when you're on board, and if it's something you're doing then he should go nicely with someone else on board - plus, you'll be able to see what he's doing with a rider on top. Try lungeing him over some small jumps to assess what he's doing from the ground. Wen you're jumping build up very slowly, when you make a double, put the pole right behind the other and gradually move it away inch by inch. Doubles and bounces require more momentum, agility, thought and oomph than simgle jumps so make sure you've got what you need there before you start. Try putting placing poles before and after the jump so he gets used to going over a number of obstacles instead of just the one. Sorry about the essay! Just try lots of different things, and take small steps, and establish a good relationship on the ground so he trusts you.

I would be very careful about taking advice from anyone other than a qualified natural horse practitioner or equine behaviourist. There may be other issues as to why your horse acts in this way that are related to tack or health issues. Is he comfortable in his mouth? Does he has a healthy back or is his saddle causing him concern? In terms of behaviour I would arrange a session with someone from the Intelligent horsemanship association. I had one in my area come out and the insight to my boy was amazing. She picked up on things that I had taken for granted ( a flick of the ear, posture, positioning etc) and explained the meanings in the horses terms. It was amazing nad took me on such a journey with my boy. We are now in such a better place and I would recommend it to anyone !