My 9 week old GSD adores his kong. Will go RIGHT into the kennel if he sees I have it. However all he does is chew off the part of the treat that sticks out. Once that is gone, the kong is dead to him. I've tried also stuffing peanut butter into the kong but he won't ever try to get it out. He sniffs it, knows there is food, but gives up after two seconds. Is this just an age thing or are kongs just not for him?