Do you want to know a cue to use on the lunge or under saddle?
On the rare occasions that I lunge my mare without a surcingle, she will sometimes do some messed-up gaits because she doesn't have me to help her hold herself together. I personally like to do this every once in a while... to teach her to hold it together on her own.
Does your horse know verbal cues for "woah," "walk," "gait," "canter," "speed up," and "slow down"? If not, I would definitely teach him that.
My mare knows the words "woah" and "walk." I use "trot" for gait, and I cluck quietly to tell her to speed up and cluck loudly for canter. I say "whooooop" when I want her to slow down.
If she goes from a walk right into a slow pace or a stepping pace, I say "ahh" and step out in front of her a bit. This causes her to go back to a walk and realize she did something wrong. Then I say "trot" again.
If she goes from the gait I want (foxtrot in my case, runwalk or rack in your case), it's typically because she's going faster than she's ready to go. In that case, I tell her to slow down.
If she's not cantering correctly (or foxtrotting into the canter, which I HATE), I speed her up and then let her slow down on her own. She knows when I'm lunging her that she needs to be collected at the canter and she knows if she doesn't she's going to have to canter faster.
Now if you're talking about under saddle, it'll be pretty similar. If you ask him to gait and he goes from a walk right into a stepping pace or a pace, scold him. Say "ahh" and bump him back to a walk. Then ask him to gait again.
If he goes from a runwalk or rack to a stepping pace or pace, scold him and slow him down until he gets back into the runwalk or the rack.
I never encourage horses to trot/gait into the canter. If you don't want him to do the strange gait (and trust me, it's NOT comfortable under saddle), try speeding him up to a normal canter and letting him slow down on his own. If that doesn't work, I would make him stop every time you ask for a canter and he takes a step in something that's not a canter. And then ask again...
I wouldn't encourage the stepping pace if I were you, especially if he's wanting to do a hard pace as well. It'll only make it worse.
Hope that helps!