[107barney]

You need to train him to give up the prized possession. To do this, you trade him the object he has decided is of high value with something that he would find more valuable (delicious food). I would not grab the item from him. You want to throw the item of higher value in his direction but far enough from him that you can quickly put your foot down on the item he needs to leave in order to retrieve the higher value item. I would train him to drop or give with something else -- like a toy he doesnt highly value so that he will learn to give up items. With items that are being protected as precious resources you need to tread carefully so as to not make the problem worse.

By the way, you are very smart to recognize that a problem is starting. You need to nip it in the bud or it will get worse. If it continues, you may need a trainer to help you with this. Guardiang of resources can be a serious canine problem.