Hide a treat in a closed hand. Hold this hand out, to entice your dog to sniff it. While he tries to get the treat, hold your hand as still as possible. Then as soon as he head moves away, even a little bit, quickly open your hand and offer the treat. Repeat this several times. Timing is very important here but hopefully he will soon learn that moving his head away from your hand gets rewarded.
Once he consistently moves his head away, you can add in the verbal cue "Leave It".
As a second part of this exercise, you can also use this to teach him to look at you for guidance:
Hold the treat in your hand as before. As he moves his head/nose away from your hand, move your hand towards your forehead, so his eyes move towards your face, before giving it to him. (Then eventually, if he is looking at your eyes consistently while tracking the treat, you can add in the verbal cue "Look" or "Watch").
An alternative way to do this is as follows:
Your first hand holds the treat as before, in a closed hand.
The other hand also holds a treat. Place this hand near your forehead.
When he moves his head/nose away from your first hand, as before, give him the treat you hold in your second hand, followed by the treat in the first hand (which can also move towards your forehead).
Once he knows the verbal cue, you can request "leave it" in day to day situations, for example if he starts sniffing something on the table, ask him to "leave it" then when he looks at you, praise him and provide something better as a reward.
To teach your dog that things that drop onto the floor, or are found on the ground (for example discarded food in the park), are not to be touched, follow this exercise.
First, show your dog the treat you're holding, then drop it on the floor, aim to drop it behind you so when he runs at it, you can block it with your body. At the same time, ask him to sit. When he does, reward the sitting, and move your body slowly to unblock the treat but continue to reward him if he remains seated and focussed on you. You can then release him with a verbal cue, such as "Ok" or "Free" so he can get the treat you had dropped. Repeat this exercise in short sessions, rewarding him to remain seated and focussing on you.
If something drops on the floor at home, ask him to sit before you pick it up. Then reward him for sitting patiently with a treat. This way he will learn to automatically wait when he sees food and realise that things on the floor are off-limits.
Good luck with these exercises! Let's hope they are fun for you and your pet :)