STEP 1: TEACH BEHAVIOUR
Hold the treat in your hand, put your hand in front of the dog and wait for the smallest step of your dog losing interest in a treat. The moment he looks away from a treat or starts doing anything else except focusing on treat click wait a few seconds than treat him.
STEP 2: ADD CUE
Repeat step 1 but now introduce a cue, like saying leave it. You can increase the difficulty by holding your hand with a treat open. It is important not to let your dog get the treat so if he moves in for a treat close your hand.
STEP 3: INCREASE DIFFICULTY
Drop the food on the floor and say the cue. Make sure you are close to the floor so you can cover the treat with your hand faster than your dog can grab it. Again click and treat when he loses the interest.