Slowly socialize your dog with others. The only way to train a dog to not attack other dogs is to socialize him properly. Start by introducing a strange dog at a distance that causes no reaction, ensuring the dogs know of each other’s presence. Praise the dog for not reacting. Over the next few weeks perform the same action, while decreasing the distance between the dogs slowly but steadily. Use a different dog each time, so your dog knows that it is not okay to attack any dog, whether he is used to it or not.