Ha had to laugh at Witcherys comment. Yeah I wouldn't round up cows if I were a horse. I'd be far too fat and lazy!!!
Back to the subject, so many factors could be affecting your ex-racehorses inability to round up.
How many starts did he actually have? If it were quite a few then he will be hard to re train if you haven't re-educated one before. Try a lot of walking excercices. 10m circles and then out, try getting him to flex inwards. Just little excercises to get him soft in the mouth again and supple through the neck can be a good start. Dont try to get much out of a trot or canter if he cant do it at a walk. Then when he is ready slowly ask for more contact. It'll come Bear-luva, it will just take some time.
As they say, Rome wasn't built in a day!