I have a 7 year old OTTB. When we first started our dressage training, the things I most had to work on were getting him to move forward off my leg, and getting him connected properly over his back. If you have a dressage trainer in your area, I highly suggest taking a few lessons so you can a good feel of what proper contact feels like and also have someone to help you with the forward part. There's also a website called the Art of Riding (I think) that has all of the concepts broken down into plain speak. I'm sure you can find it on google. Just know that when you start asking for you OTTB to start using himself correctly, he will be using all new muscles that he didn't use for racing, so he might have some stiffness from time to time.