Teaching someone elses horse to go at YOUR pace?

I would let him move his feet, but the direction should be chosen by you. Turn him around, a 180 turn from where you were going. Take off the side of the path and circle him around or figure 8 across the path for a bit until he is calm. Then go forward on the trail again, ask him to stop and back up a few steps. Keeping the feet busy keeps the mind busy and you have to control the mind or you will never control the feet. Just find some little tasks for him to do and then move forward again. If you are consistent, he will be better and listen to you. You have the right idea, but if the owner is also riding him, it will be pretty much a constant thing to have to remind him that he must be accepting of a change in his usual path. Too bad the owner does not ever vary his routine. If that is something the owner would consider and you feel comfortable asking, it would help you and the horse. Personally, I don't do the same thing repeatedly. We have a different group of tasks to accomplish every day so my horses never know what to expect so they don't get into a rut. I always dismount at a different place. Horses learn quickly and are very smart so as humans we have to work at keeping them paying attention and happy in their job. You will do great. Sounds like a nice horse.