Along the lines of the positive reinforcement suggestions, have you considered trying clicker training? You can click (and immediately treat) for any movement of the back legs and then shape it into a full movement. Negative reinforcement (removing discomfort) is effective, but as others have mentioned we often end up desensitising our animals (I work with dogs) to increased pressure. Meaning if you have a willful horse, you'll always find yourself having to increase the amount of force you use in order to achieve the desired finished behaviour, and once he gets used to that, you'll have to increase it again. Doesn't make for very fun training for you or the horse. I think in order for training to be effective, it has to be enjoyable for everyone involved. Pain is definitely a motivator, but it isn't the only motivator. Find out what the horse likes and make him work for it.
The thing about clicker training is timing. The better your timing is, the clearer you will be to your horse, and as you learn with it, the more behaviours you'll be able to apply it to and who knows, you could very well end up with a horse who is offering all types of desirable behaviours because he has learned to enjoy training. If you're interested in it, try and find some dog trainers local to you that use clicker training - oftentimes they will have worked with horses as well, or will be able to refer you to someone with horse experience. But since all animals learn the same (research "four quadrant learning theory"), it doesn't matter whether you're working with a horse or a dog. Heck, one of my mentors clicker trained a goldfish!
I agree that as a necessary first step, you should get the horse vetted to make sure he doesn't have any pain or discomfort.