If you've never commanded down, it's not hard to teach, especially for snack lovers. Grab a tiny training treat. Make sure your dog sees it and have him sit. Say "down" (or "lay," if you'd prefer) and simply move the hand with the bit towards the floor so he naturally has to move his head down and paws out, too. After he consistently performs with word only, use the position to begin teaching roll over, play dead, crawl, and other ones that start out this way!