Racehorses have no need to be taught to respond to the legs, particularly legs hanging around their belly, hence very few of them will move off the leg when coming ott. I would start out by doing a lot of ground work and lunging. Once they understand voice aids in hand and on the lunge, it is much easier to teach them to respond to the leg. When working with the ottb's I've had/worked with, including my current boy, I always start out by teaching 'walk on' and 'woah' in hand. I want to be abel to use my voice for both of these commands without having to pull the horse around.
From there, I begin lunge work, initially keeping the horse close to your side and slowly allowing it out onto a larger circle. All the while working on 'walk on' and 'woah'. You can start trotting, from 'trot on' or whatever voice command you wish to use, and 'woah' to halt again.
I generally won't get on an ottb until they will walk, trot and halt from voice alone.
When it comes to getting on, do it on the lunge. Put the horse in a round yard, lunge first, getting them responsive to your voice again before you get on. Have an experienced horse person stand in the middle to lunge you on the horse, so that they have control of a lunge whip if the horse won't move off from your aids.
Begin by asking the horse to 'walk on' with your voice. Allow the horse to move off, walk a little then ask for 'woah'. When he's comfortable with your weight on his back, and doing basic walk-halt-walk transitions you can start to put your leg on in the halt-walk transitions. So come back to halt, put your leg on lightly and if he does not step forward, say 'walk on'. Do not start kicking wildly as soon seem to do when the horse does not respond. He does not respond because he does not understand yet. It's like yelling English at a Chinese person who cannot speak a word of English - yelling won't help them to understand! Instead, ask your 'helper' to flick the lunge whip towards his quarters. Hopefully, you have established that a flick of the lunge whip behind him means 'go forward'.
When he steps forward, take your leg off and allow him to walk. Keep your reins loose, you absolutely do NOT want to gob him in the mouth if he jumps forward! This will just teach him that going forward is the wrong reaction.
It doesn't usually take an ottb long to associate the leg aid with 'go forward', they are quite smart in general. When he'll start moving off with your leg (keep using your voice as well, it is a good confidence builder for him ;)), you can start working off the lunge, in a fenced in area (60x20m sand arena is ideal) and concentrate on the same thing - halt-walk-halt with very light leg and rein.
When he's comfortable and responding well, you can start to use the same technique to teach trot from the leg aids.