I would suggest just letting him canter. Pick a spot on the horizon and go for it.
Or, if you only have access to an arena/roundpen, do laps at the canter. When he starts slowing of his own accord (and he will), praise him while he's cantering
(it's the slow canter you want to reward, not the downshift to walk or trot), then let him come to a walk and rest, and catch his breath.
Be sure to balance this with transition work to keep his impulsion up and his weight back.
Cantering just half the arena will make him worse, because he'll start anticipating the starts and stops, and that the canter trip will be short. So many lesson horses get wired about cantering, because they only canter for short bursts for the benefit of learning riders. When they don't know whether you'll be cantering for 30 seconds or 30 minutes, they slow down pretty quickly.
My only caveat with this is that with his being 18 years old, and my not knowing how gracefully he's aging, you may want to take some precautions with his legs and joints. My sister will literally lope her 14 year old QH for 10/15 minutes at a time, just a pleasant, ground covering, controlled canter. That length of constant canter may be a bit taxing to an 18 year old's joints. I'd recommend support boots, perhaps a maintenance joint supplement, depending on your horse.
Good luck, and welcome to the forum!