Teach your dog the "down" command when he is in the sitting position. Step to the side to give him room, then use your hand (with a piece of hot dog) to guide him down to the ground with a sweeping motion while you say "Down." He should follow the hot dog with his nose as you stretch your hand down to the ground and out in front of him. Push down between his shoulder blades to show him what you want him to do. Then reward him by dropping the treat between his front legs.
This will take a lot of repetition, but he will get it if you are consistent. This command comes with an "automatic stay," so he should stay in the down position until you release him with the command "OK!" Keep repeating the word "down" while he is in the position, so he will know you want him to stay until released. When you say "OK!" it should be in a very upbeat, happy voice.
Mastiff - Dog Lover