If you are comfortable riding him in general then the height difference shouldn't be too bad. However, if you are at all hesitant or feeling unbalanced then your horse will be able to pick up on that. Just relax and try to go with it no matter how bad it is. If you have someone on the ground just casually watching with a lunge whip that might help if the horse is reluctant to pick up the canter. The biggest reason a horse can't/won't pick up the canter is because it doesn't feel balanced. So for this reason you should start picking up the canter on the straight sides of your arena, preferably coming out of the corners.
Just work your horse for a while, until he's in a good state of mind and is working forward and energetic but not too crazy. Then just sit the trot, slid a leg back and push with your seat. Chances are he's just gonna trot faster. Bring him back to a regular trot then ask again. You can add a tap of the whip. But at this point you need to just get the horse so that he understands what you mean when you ask for the canter. So just push him until he canters then you can pat him and make a big fuss over him so he knows he did well. Don't ask him to go for long periods of time he will get tired and it will be harder. But little and often is best.
Like I said, if he understands lunging he will respect the lunge whip. Just having someone on the ground carrying a whip, perhaps flicking it towards him when needed...can be a huge help.
The worst thing you can do is tense up. You are going to at first, understandably, but just think relax your seat into the saddle, sitting up and sitting back a bit. If you get at all forward he will feel your weight shift and trot. You should try to remain neutral as possible, remember it will be easy for him to fall off balance.
Also! If you ask him to canter, and he does, but he then breaks into the trot..immediately put him back into it. He needs to learn that only you can ask him to trot...he can't just break whenever he wants.
Good luck! Just remember! Relax, head up, sit up tall, and go with it! Might not be pretty at first but hey! Its a work in progress:)
DRESSAGE, n.: the passionate pursuit of perfection by the obsessively imperfect -Author Unknown