Practice these two exercises often so they are in place when you really need them.
Use the “leave it” command when you want your dog to move away from something. To train the command — have a helper tempt your dog with a low value item (dry food, boring toy) — when he shows interest in the item move toward him, place a higher value item to his nose (yummy food treat), and lure him away, cheerfully telling him to “leave it” as he is moving toward you. Give him the treat when you have moved him several feet away. Do this several times so that you associate the command “leave it” with the act of moving away from something.
Once you think he has the association, you can start using “leave it” as the command. When he is interested in something, say, “leave it” without showing him the treat. If he has learned that those words mean move away from one thing and get something better — he will do it. The next step will be to increase the value of the items he must move away from.
Use the “drop it” or “give” command to teach your dog to give up things that he has in his mouth. When he has a ‘treasure’ approach him with a high value treat, show it to him, and when he spits out the item to get the treat say, “drop it” or “give” in a cheerful voice as he does. When he moves away from the item and toward you, praise him and give him the treat. Then pick up the item he dropped and give it back to him. Repeat this several times in a row and then leave him with the original item to play with. This way he learns that dropping an item is rewarding (because he gets a treat) and he will often get the item back in the end.
Practice this exercise with as many items as you can so your dog understands what to do when you say, “drop it.” When you think you have associated the words “drop it” with the behavior of spitting something out of his mouth, you can start to use it as a command for the behavior.
If he is ever reluctant to drop his treasure, drop a quantity of a high value treat far enough away so that he has to get up to reach it.