Once your dog gets the hang of obedience training, you won't need to repeat various commands to him. Simple hand motions will suffice for him to sit, stay and lie down. It takes practice, perseverance and a good supply of treats, but he'll eventually succeed.
It takes a lot of practice before your dog recognizes only the hand motion and doesn't need the oral command. If you're participating in a weekly or biweekly obedience class, make sure you practice the exercises with your dog for at least 20 minutes a day outside of school. Before learning "down," he must reliably know "sit." Start teaching him "down" by having him sit. Verbalize "down" while holding a treat in front of his nose. Move your hand toward the floor, so the treat is between his front legs. His nose follows the treat. Keep the treat between his paws, then slowly move it away along the floor. Your dog probably will lie down to follow the treat. Praise him and give him the treat. Repeat this practice regularly until he lies down on his own upon command.
When he's consistently lying down with verbalization, start using the hand signal for "down." Bend your right arm a bit at the elbow, holding your fingers straight and palm outward. Make a downward sweeping motion. While you're not using a treat with the hand signal, have one available so that the moment he lies down, he gets a treat and praise.