First off I realize in the pleasure world this probably isn't true but a lope and canter should be synonymous.
Ruling out pain (Some horses will go faster trying to get away from pain from saddle fit, teeth, the works) start him loping, lope for a few strides, then break him back down to a trot for a bit longer. Then lope a few strides, back down to a trot. This way he is automatically thinking "slow" when you get going "faster".
Pssh.I didn't pick up the wrong lead
It's called a counter canter...
...A very advanced maneuver.