If he raced, all he knows is to take the bit and run. For starters, I initially would not do anything with him dead straight. That's just asking him to brace against the bit and go. From the moment you get on and are warming up at the walk, try to ride with flexion so that you can see his inside eyelashes. Shallow serpentines where you alternate every few strides to change the flexion will start getting him to understand bending. Make sure to provide him with a nice soft inside leg to bend around. Every time he responds properly and softens even a little to the bit, reward him immediately by softening the contact so he knows that's what you want. Most TB's learn this exercise very quickly.
From there I would teach him leg yield, shoulder-fore and haunches-in at the walk. All will achieve suppleness and get him understanding bend and working from behind.
Be very clear and simple with your aids. TB's can get frustrated easily if they do not understand what you want. If he starts to get worried, just back up to an exercise he knows until he settles down and then move back onto the new one you want to teach him. Assuming you have access to trails, go out and play every few rides instead of working to keep him happy and fresh. I'm sure you'll do great with him.