Can you flex his nose around to your foot laterally on both sides at a halt? Can you step you horse over to the left with his face looking right/his body bent to the right? Can you move his shoulders/ribcage/hind end independantly while in the saddle?
If no, you need to establish all of the above.
If yes, then as you go around, tip his head to the inside and yeild his ribcage to the outside, and his hind but not as much as the ribcage. Basically, bend him around your inside leg - Your open inside rein creat the start of the bend, your leg pushing the ribcage out creates the middle of the bend, and your seatbones holding his hind in alignment create the end of the bend.If he gets sticky, use more leg and yeild a few steps off the circle to the outside then continue on.