Now here's the 'but'
When I ask for a canter he just speeds up into a power trot and starts pulling ... HARD. He'll hop a little, like he's about to transition but then will fall straight back into a trot.
I'm currently using just a regular halter and lunge line attached to the bottom ring for this, and I wonder if I should switch to a bridle. But if so, I don't know if there's a special way to rig up the lunge line.
Also, we work in an open area, and I give him about 15-20 feet of line when I ask... so I assume I'm giving enough room. It also baffles me because about a year ago I had him boarded somewhere else where there was a small enclosure that I practised free-lunging him in, and he learned to canter off the voice cue in there in both directions.. Silly me never thought to teach him to do it on lunge line though while I had the enclosure at my aid.
So how do I build upon what he already knows to achieve a canter?