My dog is possessive over tenins balls. He is fixated when I get one out, so i tried making him wait and leave it before i threw it, but he still won't drop it. At first i tried to take it from him and he got aggressive, so i corrected that, but now he just backs away with the ball. I can eventually chase him down and after offering him a treat he reluctantly drops the ball, but as soon as he gets the treat or if he doesn't get it straight away, he runs back to the ball. What else can i do?