I would teach him the ''leave it'' command, but practice it with treats first. This is how I did it. If my dog is laying on the floor I'd place a treat near him but far enough that he would have to reach it to get it. If he tries to get it, I'd say leave it. If he continues, I'd take the treat away. I'd do it again several more times until he stops lunging when I give the command. When he finally leaves it alone at my command, I then give him the treat. This way he would learn that when I tell him to leave it, he would not touch it.
I then make it harder for him by placing the treat nearer to him. Every time he leave it when I give the command, then he gets the treat. Soon, he would learn that what is his is mine. Anytime Kai has anything he shouldn't have in his mouth, when I give the ''leave it'' command, he drops it just like that. No competition, no nothing.
That is just my method though.