I'd still wait until he's older. You're not going to miss the opportunity to teach him those movements when he's matured more, so why not wait... His body is not ready for any dressage work yet. Those movements are not just movements, they need muscle and joint development, too. I, personally, would wait for it until a horse is about 4 years old. But, if he will be let to mature, then he is more likely to live a long, healthy life with many years of training ahead.
Don't get me wrong, I'm not trying to bash anything here, just my opinion. :)