If you have tried everything else suggested, and checked his tack, etc, and it just is him trying you, you can try this if he knows light pressure. By light pressure, I mean if he knows to move away if you lightly put pressure with your fingers or hand on his flank, shoulder, etc. If he understands that, and he's not turning for you, bring the foot away from the turn up to his shoulder, and press your heel against it while applying the neck rein. It's the same principle as turning with your knees, but lets him feel the pressure more. You can also do a sort of splits motion, bringing the foot from the direction you are turning up to his flank at the same time, encouraging him to move his hind end. You should not have to do this consistently, just as a "tell" if he's not responding to asking.
I recently had to do this with Hoover, as he was trying me on turning. I applied foot pressure to both his shoulders and flank while asking for the turn. Within fifteen minutes he was responding from just the reins and knees. Of course, this is from the training he's had with my instructor, so Sonny may be totally different.
"Sit tall in the saddle, hold your head up high. Keep your eyes fixed where the trail meets the sky. And live like you ain't afraid to die...don't be scared, jut enjoy the ride." - Chris LeDoux