It is possible to train a horse so expertly that one can put a novice rider on him & he'll be so habituated to complying that he'll make the novice look good; some non-riding royalty in Europe rode these horses in public parades & such.
However, the horse will test for leadership eventually & to the individual's greater or lesser degree, because it's hardwired to do so. So, in the end, the permanent rider must gain the skill to be the good leader in the partnership.
Thus, all that you can do is to train him as best that you can, & if he doesn't get the habit of compliance enough to give the other riders even a walk down Main St., so to speak, c'est la vie. The person who buys him will have to become his leader anyway.