It is common for ALOT of riders to be unbalanced when they ride. You may not of had this problem in the past with your other horses, but since this horse is young you have to be RIGHT on, no matter what! I have seen champion riders that have sloppy posture, but the horse, knows how to balance that out so that it dont look like it! Im not saying you have sloppy posture but in a canter alot of people get kinda excited, leans forward (witch is a BIG no no) & one little wrong movment can throw off a young horse who is still learning to carry himself, you, & the saddle under all his gaits. the weight should be distrubuted evenly left and right, and sufficiently toward the rear legs so the horse can easily manage the task at hand.
If you are absolutely sure it’s not your posture that is throwing him off, try lounging him with someone on him or a heavy western saddle. This is what you should do before you ever try upping the gait on a young horse. It will teach the horse slowly, how to carry, balance & withstand weight on his back at higher speeds! Since each gait has a different rhythm, each gait is balanced in a different way.