Natural horsemanship has become a bit of a catch-all term, Laurie, so I can’t be sure exactly what method was used for this horse. Teaching a horse a behavior we want replicated with more than just one person often requires that the horse is consistently handled, using the same cues with several people so that the behavior “generalizes.” This means that the horse learns that all people respond the same way, so his default behavior becomes the new learned behavior. Right now, the horse has learned that almost all people behave one way, but his owner behaves another—and he responds accordingly.
Regarding catching him at turn out—he will need to learn that coming in means he gets something really good. Every person who goes to get him should always bring a scoop of grain or something desirable. I would start with the owner using this technique (so the horse can quickly learn that what is in the scoop is tasty). After a few scoop/owner interactions, I strongly suggest a two-day “power train,” enlisting the help of three or four different folks. Begin by having each person simply walk out, one by one, halter over their shoulder and grain scoop with a bit of grain in hand. Approach the horse; give the grain, give a tiny pat, and then leave. Soon, the little pat should increase to touching with the lead rope, then placing the halter on before the food is given.
Good luck, and remember that a bit of patience and a good sense of humor will help!