I thinks it's a very good idea to make your foal more meek before starting the dressage ! A lot of people don't realize how much the initial handling of a young horse will impact its future! For desensitizing your foal you have to create a stimulus such as stroking or rubbing using your hand, you must continue with that stimulus consistenly until the horse gives you no response. If you remove the stimulus before the horse stops responding, you will be sensitizing him (this is how bad habits form). He will believe that whatever action he was taking (wheter it was a swish of his tail or a stomp of his foot … or anything) is was the correct action. But, what you wanted was no action, no response.
For example when you want to teach a foal to accept various types of brushes and to allow you to groom him. So, you start with a soft, easily accepted brush such as a horse-hair brush. You begin stroking him with the soft brush even if he walks around or twitches his skin. Once he stands still and gives NO response, you remove the stimulus (the brushing).