How to correct ball aggression with other dogs?
I have a two year old male GSD who is a wonderful dog. He's fantastic with other dogs and we go to the dog park at least 4-5 times a week. Barring any random dominance displays he gets along wonderfully with dogs of all shapes and sizes, unless there is a ball around.
6-9 months ago, he went from having little interest in balls to being crazy for them. The whole world ceases to exist when he sees one. Playing fetch alone, he usually behaves well, although we are still working on bringing it back to me (without another ball to lure him with) His favorite game is keep away, although if I'm fast enough and catch him I can completely put my hand in his mouth and remove the ball with absolutely no issues. However, if we're playing at the park and another dog comes up and takes his ball, gets too close, or even when he sometimes takes another dogs ball, he gets pretty aggressive with the other dog. It's clearly possessive behavior, as soon as he drops the ball or refocuses on the one I have (again, I always have a second...or third ball) the aggression towards the dog immediately ceases.
He shows no aggression anywhere else and this seems to be escalating.
What behaviors can I work on, at home without other dogs around, that will correct this behavior?