Actually it's not "definitely" pain. This is a horse who used to fake a limp
Well won't comment here on 'faking' lameness, but you may notice I didn't actually say 'definitely pain'(although sounds highly likely), I said definitely physical - as in not 'faking' or 'attitude'.
He does the same thing in both his forward flap jump saddle and dressage saddle, so I don't think it is saddle related.
I don't understand why you'd conclude they must be OK from that though? Could they not be both the same?(Eg. They're symmetrical & he's not?) Even if not, could one have caused a problem that carries over?? You need to evaluate saddle fit carefully & check it regularly, as saddles & horses both change over time.
I have palpitated up and down his back, curried as hard as I could to try to evoke a response and he does not flinch. Vet said just a few months ago, that he sees so many horses with sore back, but mine is not one of them.
No offense but if you're not an experienced bodyworker, fact that you have 'palpated' doesn't mean anything necessarily. Perhaps he doesn't have obvious bruises, but something is out... Vets, unless specialised in bodywork often don't know that much either. **Not assuming it's the case, just not assuming you or vet experts there either.
I've been riding the same way for years, so I'm not sure how that would suddenly create a problem.
Could be, if you have a sore knee or some such lately, or could be that combined with his imbalance or something he's done recently...
His club is very mild grade. Gotta look hard to notice it. His left hip bone is slightly higher than the right, but again, why would that suddenly become a problem.
You may have noticed, in yourself perhaps, that you can function perfectly well with imbalances for ages, but then 'suddenly' perhaps 'out of the blue' or due to a minor accident, twist, jolt or such, suddenly what hass been ignored for years hurts. Have you had a chiro vet or such out to check him out? What have bodyworkers had to say about his pelvic imbalance?
I'm still thinking teeth.
So if it's teeth, it's definitely due to pain. But what makes you think that, when you've had the dentist recently(you didn't tell us he gave a bad report so I assume it was OK?), that he does it on a loose rein, that he only does it at a canter??
every other gait and direction is perfect under saddle
Is his high foot the right? Could be a little lameness in that hoof, exacerbated when you ask him to use it like that. That an unrecognised problem has progressed to mild lameness. Also any imbalances/pressure points in a saddle are felt 3 times stronger(they did tests with pressure pads) at the canter as at other paces.