He maybe need i chiropractor my cob done the same thing and I put it down to bad behaviour then I had someone look at her and actually she had put her pelvis out silly cookie
He lunges well always jumps into canter at first but settles and transitions nicely into trot and back into canter to voice command. We've a saddle that's fitted to him and the bit is a double link snaffle. May be his age and temperament and breeding to be firery.
I agree with Alysha smith and doing some lunge work to find what is causing the bucking, as there can be so many things; pain, tickling feeling, pressure, gooding off, any prior owner issues, etc.
how is he to lunge?
are his transitions the same or easy?
His back is fine not sore. Yes we stop and start again he works beautifully in the canter it's just the initial transition.
Get a chiropractor out. If he's being evasive, big sign he could be in pain in the chest or neck area.
Have you tried bringing the horse straight back to a trot to settle and asking again and repeating this?