Horses that change canter often have balance issues. He is probably very sensitive and must feel when your move your seat a bit.
The first thing to do is to do flat work, work the canter on straight lines. Then you can add poles and work like if you were doing a jumping course. When he really is stable with the poles; do the same with small jumps.
Depending on the horse, it will take a certain time and you will have to be very precise in everything you ask.