Pup Starting to Show Ball Aggression

[mjbgsd]

My oldest can get possessive with "his" toys so I don't bring his ball to the dog park on busy days. If it looks like there are two many dogs, his ball is put away and we can play fetch at home. He doesn't play with other dogs or go after their balls so he just trots in circles around me. If it's not too busy we go in a corner where there is no one and I'll throw the ball then but most of the time I don't bring him because it's a very hilly dog park and it's hard on his arthritis.