if he is not supple, then work at the trot and work on suppleness there.
Do circles at trot and get it GOOD, with correct bend, no leaning, and not throwing the head up.
Do lateral movement, such as spiraling out, or leg yields along the long side of the arena. When this is good, try this:
Go anti-clockwise (to your left), go down the long side, but be about 15 feet inside of the fence. along the way leg yield him out to the fence so that he hits the fence just before the corner, as he steps into the corner, just ever so slightly kiss to him and touch him with your outside leg, and see if he won't just slip into a nice left lead canter. When he does, let him just canter on , or drift down to a trot if he prefers, as his reward. Praise, go back to trotting and other relaxed lateral movements.
rinse and repeat. No stressful arguments!