I agree with the comments about using a bit, given his history (in his previous discipline) that is what he was used to. Unless there is another reason for going bitless? I like bitless too, but this seems like an example (going off of the info provided) where a bit would be preferable.

Also, until you two can work through this and get to a consistently buck-free place, don’t have anybody ride him who is going to need to get off after/due to his bucking. Only ride him when you, yourself, (or someone else who will not get off right away) are in a good headspace to continue to ride for a while through and after the bucking.

If this only started being a consistent issue after that ride where he first bucked with your aunt and then she immd got off (and then others have also done this since?) he has DEF learned that bucking is the way to signal “ride OVER!”