A 'lazy' horse isn't particularly lazy - he just doesn't take your aids seriously due to a lack of efficient training and respect for his rider.
Remember, horses will do what they do for a reason, and it is more than likely that he drops down to trot because he doesn't respect your aids or you are sending him confusing signals, such as:
- nagging with the leg (your horse will become dull if he is being squeezed every step)
- pulling on the reins/having reins too short (this communicates to the horse that you want him to slow)
- lack of balance in the saddle (uneven weight distribution in the saddle will knock him off balance, causing him to drop to trot)
I'd take him back to basics and begin retraining him by lunging him. When lunging, ask him for trot using a voice cue of your choice. If he doesn't respond (or takes 3 seconds to respond) get after him by whipping the ground which puts pressure on him to move his feet. As soon as he moves up a gear, stop whipping the ground and allow him to trot. Bring him back down to walk and when you're ready, then signal to him to trot. Again, if he doesn't trot when asked to do so, spank the ground with the whip and stop immediately after he picks up a gear.
He will eventually learn that the voice cue means go and if he doesn't move then he will be chased by the whip.
Same goes with cantering.
Once you've got your voice cues sorted and he responds to them well, use them when riding. It will eventually abolish the need for a crop in the ring!
I can assure you this method works if done correctly. My mare moves immediately when asked due to me using this method when breaking her in.