Retraining an off-the-track Thoroughbred requires skill and knowledge. You either need to have both or work closely with someone who does. The life of a racehorse is very different than the life of a sport horse. The aids of a jockey are not like the aids of a dressage rider.
Understanding that a former racehorse does not understand what we are asking and in many cases the training must begin at the beginning of training and it takes patience and time - a year of basic training is typical.
You will need time to build up new muscle groups. His body will change in shape during that first year of work as a sport horse. You cannot rush this process.
Here is a well-written article published on the Dressage Today website that explains the process. For even more knowledge, read the book the article was adapted from Ride Better with Christoph Hess by Christoph Hess. The book is available at HorseandRiderBooks.com.