Teaching your dog how to stand on all fours when commanded isn't only a great trick, you can use it for emergency stays, to teach him Red Light/Green Light (another great safety skill), and it's simply a great way of pushing him further in his training process.
To start, I recommend grabbing some of your dog's favorite training treats and your training clicker. You'll have your dog either sitting or laying down to start. Then, you'll want to put your hand in a fist with the treat in it. With your fingers facing your dog's nose, you will then lift your hand slightly up and towards yourself. Be sure to not move too fast with this.