Hello, Jessica. Some horses are simply more comfortable in one gait versus another. I would have your vet do a full exam on Harry, just in case there is a physical issue—and if none are found, I would start back at square one teaching him to canter (or lope).
Be sure you choose a very clear cue. I suggest a verbal cue such as “hup” and a leg cue (when you are working him on his back). It is usually easiest to teach a lope either in a round pen or up a hill, and I am going to assume finding a hill will be easier for you than finding a round pen.
Trot him to the base of the hill where the incline starts, sit forward a bit in your seat, cluck, say “hup” and apply leg pressure behind the girth on one side (this will be his cue). Your leg pressure will do two things: it will move his weight away from your leg, making it easier for him to pick up a lead, and it will motivate him to want to “turn it off.” If he trots faster, simply apply just enough pressure on the reins to stop him from being able to increase his speed. Be patient and let him think it through. If he lopes just one stride, be sure to stop the leg pressure and give him a bit of a pat. If he breaks back to a trot or walk, no worries, savor the win, and go back to the bottom of the hill and try again.
By clearly communicating what you want, you will help him immensely. Be patient and ask for more and more strides over time, keeping in mind that he is building a new muscle set and will need time to do so.