I agree with what was said - another thing you can do while riding or even lunging him is to set up a cross with poles infront and behind so it's a pole, bounce, cross-rail jump, bounce then pole. Bascially set it up as putting up your cross-rail jump, taking three to four steps out (start off at four, then set it back to three when he becomes more expirenced) and place down a rail from the jump both on the take off and landing side.

Then go through this and slowly raise the sides of the cros from so it gets tighter where he has to jump - this will teach him not to swing his legs, not to drop his legs, not to cross or twist etc etc and put his front legs, and even hind legs, into a better position over the fence.

I had a horse who used to twist and drop a leg something cruel - done that for a while and it "taught" him what position to go over the jump in. And because he got stronger at it and it became easier to jump that way instead of twisting etc

Hope that helps.