well, that one sure stumps me...*sigh*....one of my dogs would definately try to attack other dogs, but whenever she goes for it, i can just call her back and she will come...i know something that does work, but it wont unless you are near ur house...if your dog attacks another dog, u could spray him with the hose...they hate that lol....but that will not work unless you r near ur house, so im no help lol....another thing i read about is to take him to a place that usually has a few dogs, and keep in a crate so he can watch the dogs go by and realize that he will never be able to get to them and get used of it after time.....but the first thing u should do is realize what kind of aggression he is displaying...ive read about four types of aggression: fear aggression, teritorrial aggression, dominance aggression and predatory aggression...maybe you could talk to Jacks dogs trainer about what u think might be bothering jack, so u could both work it out?