WV, congratulations! Lots of praise for even one or two strides of canter and his learning will accelerate, as I am sure you know. I have had several ex-pacers and found that once the light was 'switched on' and they realise what you want them to do, they will try their hearts out for you. Switching on the light could sometimes take a considerable time, though!
I found it best to slow to walk after a very few canter strides, especially for the horse who has had problems producing a canter, as canter is tiring and disorientating at first, and you do not want your pacer to start to offer those confused mixtures of gait that you sometimes see under saddle, and which are so uncomfortable to ride. Other people like the horse to maintain the canter for longer to confirm in the horse's mind that it is doing the right thing and to reinforce the gait; both methods seem to work as far as I can see.
I think many people are unaware of the fact that pacing is a hard-wired movement - as is left-handedness in us humans - and instead, think it is simply something that the horse has 'learnt' to do, perhaps as an evasion or disobedience of some sort. Mind you, even if it were, I am dismayed that anyone would suggest progressive whipping and kicking as a solution ...
It used to be thought in Britain, and many other countries, that being left-handed was a sign of association with the devil; even nowadays, left-handed children are severely punished in some countries worldwide for merely persisting in using their left hand for routine tasks. I am ambidextrous and as a small child, was gently discouraged by my parents from using my left hand for writing and painting, as my father had suffered greatly in the 1920s as a left-hander. Prejudice against left-handers was still common, especially among older teachers, when I was at primary school in the mid- late 1950s. Fortunately in most civilised countries, the only problem for a left-hander are those posed by living in a right-handed world. I am fortunate in being ambidextrous to a fairly high degree, so the difficulties of the right-handed world are more of an annoyance and an inconvenience.
I suppose, then, with such lack of understanding of ourselves, it's hardly surprising that some people still think that pacing is a 'learnt' gait, and hence can be 'unlearnt'. it is just a pity that those people haven't bothered to unlearn some of the outdated nonsense they use as an excuse to bully horses into unwilling submission.