Overprotective of his toys

Houston is a 4 year old cocker spaniel. We have had him for almost 3 years and over time he has become more protective of his toys. If he has a tennis ball, he never puts it down. He will whine, walk around the house, sit, and sleep with it in his mouth. He is that way with his black Kong too. If we put a cookie in the Kong he will growl. Even if Kenny my older dog walks by he will growl like he thinks Kenny is going to take it when he isn't going to. So we rarely put a cookie in his Kong.

I would like to lessen this behavior. Any suggestions please.