I'm not trying to offend the OP but shouldn't he have basic dressage training down if you are trying to jump him? I mean that with all kindness, I'm just confused as to what he does know or what your definition of basic dressage is. If you want to give up jumping and practice basic dressage moves there are a gazillion at the walk. Walking squares, where you do a turn on the haunches in two corners and a turn on the forehand in two opposite corners, or using barrels or poles/cavalletti's which testing your steering ability. I used to do this one where I get my horse to put his front two feet over a cavaletti pole on the ground, halt, then try to side pass/leg yield across it, so that without stepping/backing over the pole again. Eventually the poles not under you anymore because you went sideways over it, at the walk of course. I'm wondering if your horses basic dressage training or lack of might be why he is refusing? If he doesn't understand what your leg really means solidly, he doesn't feel supported at the base of the jump by it, so he feels kinda abandoned/clueless in front of the jump. I'm not telling you to continue jumping, but maybe if you really invested a six months or a year in solid flatwork you might be pleasently suprised if you reintroduced him to jumping. If you skipped building his confidence in your cues, then dressage will work wonders, but if basic dressage means more advanced then what I'm describing, and you feel like he does hate jumping, then good luck with your change of discipline, they are all great for some horses, and some horses are great at all of them.
Troubled TB ~"A thorn by any other name will prick just as deep." @-'--,---