I have a horse like that. Jester is the laziest horse on earth and my legs got an extreme workout from having to urge him on every other stride. I finally gave in and bought spurs. Coronet Humane Spur Set | Dover Saddlery
These are very gentle (unless misused) and worked wonders. I would touch him with them when I could feel him about to break into a trot and he'd keep plugging along.
Now, be careful and work on your horse's strength and fitness before just going out and buying spurs. I knew it was a case of laziness with Jester because we would go on 12 mile trail rides and 10 mile rides on the beach, so I knew it wasn't a fitness thing with him. If you are confident it's just laziness and there is nothing physically wrong with your horse, then the spurs would help.
Also do some exercises to help build up muscle. Some good ones are trotting up and down hills, and trotting poles. Doing things like that will help your horse maintain the canter better and get into shape.