Do you have a video available? I'm going to guess that your pony knows he's supposed to change his lead but he's too unbalanced to get it so he's speeding up and hopping to try to get impulsion to get the job done. When you land make sure that you use lots of half halts, leg yield, move his haunch over, etc etc, whatever it takes to get his body back to being balanced and straight, and probably off the forehand, again. Then the changes will start coming. The best way to do this? Flatwork flatwork flatwork! Get him straight and balanced on the flat, then once you start jumping he will be used to being straight and balanced, or it won't take too much work to get him there.