Hi everyone! I like bringing my dog to the dog park, as it's a great place for him to run off-lead, practice his recall, and let him play with other dogs. Today however, someone brought their young puppy in and he went after it. Didn't do any damage, just pinned him to the ground and really scared the poor thing. He's done this one other time, again with a younger puppy. I've never seen him be aggressive toward an adult dog, even when other dogs were being aggressive toward him. Before this he was running around with other (adult) dogs and playing happily. He's only a year and a half old himself. How would I go about correcting this behavior?