AOT is amatuer/owner/trainer
So experienced amateurs?? Is that a contradiction of terms?
I would guess he hasn't been treated &/or trained very well. Without more info about the way these people handle him, how long they've had him, can't even guess as to how responsible or otherwise they may be for his behaviour.
I would 'start' this horse again, from scratch. To confirm what foundations he has, or to teach them, before moving onto the 'upper levels' such as riding & eventually showing. I think the horse is very likely lacking in a lot of foundation training, including trust in his handlers. The people are obviously not up to this without help, so if they were my friends I'd offer help or recommend a good trainer/instructor.
Showing & the likes should be more of a final goal, the end result, rather than something that happens regardless of the horse/rider behaviour. Therefore it is not a reason not to go bitless to begin with(tho of course there may be other reasons... See Dr Cook's site for more reasons why you might want to). If you want him to get over his troubles with it, it is far more effective not to force the issue until you've got him over his fear of it and can teach him *considerately* to accept it.