The man I learned to drive from, would hitch his horses to something very, very heavy, that they could NOT run with. Pull, yes, but not run away with. Perhaps that would help you. Don't let him feel the weight of the shafts until he's hitched to the cart and attach something heavy, like a large log or a telephone pole, to the cart. That way he can't run, and will have to deal with it.
Looking at the trail through the ears of my horse...wonderful!