Very lateral Walker very often have trouble with the canter. If he has any show background he may have even been punished for moving from the running walk into the canter.
A true canter is a three-beat gait. So the horse has to learn how do move out of the four-beat movement.
One way is to teach the horse to trot. This will break up the pacy, four-beat gait and permit him to try and find the three beat canter. This is best done by somebody who knows what they are doing. Doing it from the ground is quicker, but requires more skill.
While this is heresy to many in the gaited horse world it can be an important thing to do. Review the thread on Gaited Horse Dressage and get an opinion as to why.
Another option is to teach the horse to work on the bit and then engage the back end. Do this in a snaffle 'cause there will be some significant contact involved until the horse figures out what you want.
First, you must ensure that the horse will yield to the bit. If they fight the contact you'll get nowhere. Once the horse yields you may then use the leg to push the horse more firmly into the bit. Remember that you're taking the horse to the bit, not the other way 'round. As the horse accepts this higher level of contact you'll see a yielding at the pole and the back will begin to rise (to bascule). It's common the Dressage/dressage world. This will also push the gait away from the pace and towards the trot. Some horses require a lot, some not so much. This exercise is far more about the leg than the hand. Once you get the horse to work in a reasonable bascule you should be able to strike the canter without too much effort. If you have to push the horse into the canter then you've not done it right. Usually timing is the problem.
Last, and far from least, the horse must be fit enough and strong enough to balance itself and hold the canter. If it's not then spend a long time walking up and down hills, intersperced with short (but steadily increasing) time at the gait. This will build wind and strength. It can take several weeks of steady work with a stalled horse; less with a pasture horse.
The "quick fix" here is to teach the trot from the ground. The longer fix is to teach it from the saddle.