...but instead of holding him together as an English horse, I let him find his own headset...
If he finds it on his own, it isn't 'headset'. It is balance. Or perhaps it is where his head ends up when balancing and moving in a way you enjoy riding.
But in western riding, the goal is not a near vertical face breaking at the poll, because it all depends on what you are doing, and the horse gets to figure it out for himself. It is related to vision, since the horse has a fairly narrow field of view with binocular vision, and has to adjust its head according to what it wants to see well. Thus it is related to speed, since a horse moving fast needs to look carefully a further distance away than one moving slow.
The neck, likewise, moves as the horse chooses to achieve balance, or sometimes relaxation.
When I think of headset from a western perspective, I think of folks see-sawing on the reins to get a look desired in WP or reining. Or I think of the folks who have read a 'dressage for dummies' book and want to 'collect their horse's head' - which, of course, is abominable pseudo-dressage not related to real dressage teachings.
If Mia stretches out too much at a canter, I'll bump her reins, or maybe lift them a little and hold until she figures it out. Then she lifts her head slightly, maybe changes the angle of her head and shortens her stride. Then we are back on a loose rein, with her carrying her head anywhere she needs to for her satisfaction. I'm not asking for a 'headset', but a change of balance and stride.
Someone competing in arena competitions in front of judges may have a different view and a different goal. My comment was for the bulk of western riders. I don't think the bulk of western riders are found in arenas.
I believe my original statement remains accurate: "The norm in western riding is to drop the idea of 'contact' and headset. Ride with slack in the reins, and let the horse choose its head position.
I cannot speak to western riders in Canada, or how they ride. I'm in Arizona, and have friends who ranch in Utah. And if anything, I think the absence of 'headset' from their vocabulary means I hang out with folks I can be proud to know and ride with...
BTW - I still like this comment from the non-western rider George Morris. I still like a forward seat, but don't tell my wife - she bought me the used western saddle I'm using now as an early Christmas present.
In fairness, there is also a valid western tradition brought over from Spain via Mexico & California, which is the bridle horse. It uses a very different approach that is training intensive. The woman who did so much to help Mia is now taking a 4 year program to learn about it.