I'm glad to hear he was given six months off to rest, relax, and lose his racing muscle. There's nothing more frustrating than trying to retrain a horse in peak racing condition. You need to take things slow, and an extremely fit horse will be frustrated by this.
I have two OTTBs. My 2003 mare came off the track with incredibly balance. This is a mare that knows exactly where her feet are. Leads and flying changes were never something I had to work on much with her, because they came so naturally.
My other OTTB was a tall, leggy guy and came off the track with little balance and a not-so-great sense of where his feet were. Even three years after coming off the track (he's 8 now), he occasionally picks up that left lead when I ask for the right. He favors his left side.
I spent hours working on his suppleness and bend to his right. Work on two-tracking and leg yielding, teaching him to move off of pressure behind the girth and at the girth until you can control bend throughout the entire body. Remember that bend isn't a quality of the neck, but of the entire body. Do lots of spiraling in and out at all three gaits on a circle, leg yielding in and out on a circle, etc.
One exercise that really helped my gelding was working on a circle in the direction he's uncomfortable with. At the rising trot, start by working with an exaggerated bend to the inside and really wrapping him around your inside leg. Every few strides, change your bend to the outside, exaggerating his bend around your outside leg and sitting a beat so you're rising when his outside legs are together. Every few strides, alternate which bend you're asking for. When he feels supple, ask for the canter on the correct lead when he's bent to the inside.