When you typically teach a dog to lay down you use a lure to get them down to the ground. The problem then is you have taught your dog that in the visual cue for laying down is you putting your hand on the floor. In order to teach your dog to lay down while standing up you have two options, the first is to ask for the down using the lure about 10 times, then stand there and wait. If your dog lays down, reward him with the food.
The second options is to fade out the lure by slowly raising your hand each time. See my video for how I demonstrated this with my 4 month old puppy.