You could try doing some small circles inside your large circle. Start at the trot and every time he drops his shoulder, take up the inside rein and help him pick up the inside shoulder. Use your inside leg to help with the bend. It won't take him to long before he starts balancing himself.
Once he can balance in the trot then you can try in the canter. Don't worry if he needs to break to the trot for the first little bit. You don't want your small circle to small, otherwise he won't be able to maintain the gaits.
Also the other thing you might want to consider is that he may not be capable of bending properly. A lot of OTTB's could use chiro. Most have issues through the lower back and hip area.