I have a 6 month old GSD and he has begun to display dog aggression about one month ago. We socialized him and he seemed to be fine with other dogs before this. Along with my mini aussie, we have been taking him to the dog park in the secluded area to get used to being around other dogs but still have that barrier. He continues to bark and growl at other dogs when I take him for walks around the neighborhood. I would like to start him in Schutzhund but they will not take him with dog aggression. Any tips? This is my first GSD.