It sounds to me like he's probably lost some of his conditioning. Canter takes much more fitness to do slow and balanced than to rush around and you say that he's had time off, so that very well may be the case. The best thing to do is lots of walk trot transitions, lots of trot work in circles, changing directions, over and around poles, up and down hills (trotting down too!) and more and more transitions. Once he builds more muscle, then add canter transitions. A great exercise is canter for a few strides then right back to trot. Sure, it sounds easy enough, but in all reality it takes a LOT for a horse to go from trot to canter back to trot in just a few strides, and back to an ENGAGED trot, not a fall on my face plod around trot
Good luck, and keep focusing on that trot work. I've seen results in as little as a few weeks. Remember better canter doesn't come from more canter, better canter comes from more trot work!