You should really bring him back to step one, it kind of seems like you've rushed his jumping a bit. I'd place a jump pole on the ground, and canter over it, see how many strides he takes before he gets to it. Then once you get a good idea of how many he would naturally take, tweak it a bit. Like see if you can lengthen or shorten the amount of strides before he gets to it.
Then you could try setting a very low jump, like one foot, and place a ground pole one stride before the jump and one stride after the jump. It will teach him to measure the correct distance as well as teaching him not to rush jumps.
Hope that helps :P