If your dog consistently expect to receive a treat at your left side in heel position he will gravitate to being there.
Practice delivering treats to your dog in this spot. Start with a closed left hand full of treats. Hold it at your belly button. Deliver treats one after another to your dog in this position moving your hand from your belly button outside to your hip and then down to your dog’s nose. Then drop a treat on the floor and take one step to
the side. Keep your hand at your belly button. If your dog has an expectation of getting treats at your left side he will move back into position on your left side. When he does, click/yes and starting again with the same motion on your left side.
Start practicing this drill in your home and then do it again in your yard and then outside.