Thank you for that information. It is helpful. Probably the first thing I would do is have him checked for a thyroid condition. This can cause a dog to be aggressive. There are other medical cause for aggression as well and they should be ruled out as well. You can read more on this here:
Let's go over why dogs are aggressive toward other dogs. Often they are fearful of other dogs and thus are aggressive before the other dog can be. In other cases, a dog is aggressive in order to dominate the other dogs and be the alpha member of the pack. Other causes could be that the dog feels they are the alpha member of the pack and as the alpha member they must protect the pack (you) or their territory from threats (other dogs).
Owners sometimes make the situation even worse by tensing up and worrying about what will happen. The dog senses the owner worry and feels that he is justified in his aggressive stance because you are obviously worried about the dog. They don't know you are worried about them attacking, they just feel that you are worried and assume it is the other dog.
I don't positively know the reason why your particular dog reacts in this manner, but I suspect it may be fear based or dominance based. Many dominant dogs are described as well behaved until you try to get them to do something they do not want to do, and then they reprimand you either with a growl or bite if you don't heed the growl. Things like taking away something they want, making them move when they don't want to, waking them up, etc can cause them to reprimand (bite) you. This can happen also with fear based dogs but they tend to react like this more often than dominance based aggressive dogs.
If you currently allow him on furniture, then you need to not allow that. Dogs that are allowed on furniture tend to feel that since they are elevated to your level or higher if on your lap, they mentally feel elevated as well in the pack order and thus are the boss. Keeping them on the floor can help lower them mentally back to a submissive position in the pack.
You have done a lot of work with him to correct a lot of issues but you don't mention how much obedience training he has had and what amount of training happens every day. He really needs to have daily OBEDIENCE training. I also think that once you feel he listens well to commands at home, he definitely needs to be in group classes with the trainer fully aware of the issues he has and had. Training classes are a great way of socializing dogs around other dogs because everyone's dog is on leash and contained and the owners are actively trying to control them. Your dog doesn't have to worry about another dog jumping them or sniffing him or dominating him. I'm sure his time at the rescue was likely traumatic and he possibly was attacked by other dogs. He might also not have been very well socialized as a pup either which would contribute to the issues he had when he was first introduced to your household.
The following site is helpful at teaching obedience training and it has other training ideas as well which will come in handy to keep him engaged and stimulated which is definitely needed with Patterdale terriers. Be sure and click on the link to the page on obedience at the bottom. and links on subsequent pages leading to detailed instructions.
Training works best if you train at least 30 minutes a day (two 15 minute sessions). I would start making your dog work via the Nothing in life is free program (NILF). It is outlined below.
Obedience training serves various purposes. It helps a dog learn what humans expect of them when they state a command which leads to self confidence and less fear. Each time a dog obeys a command, even if it is for a treat, it makes them a little more submissive to that human in the future which helps with dominance aggression. And since it is the leader or boss who is responsible for protecting the pack, if the dog is made submissive with training, you are responsible for protecting him, so that can reduce aggression due to fear and dominance.
Use things like vienna sausage slices and liver slivers for treats when working on serious issues. They are high value treats and dog tend to work for them and thus you achieve a change in behavior faster.
Here is the method I use to help stop a dog from lunging and acting aggressive toward dogs when on lead but it does work better if you have a helper with a dog to recreate situations where your dog reacts. What you will do is have your dog on the leash. You will have your helper off in the distance. Your helper will gradually move their dog a bit closer to you preferably walking past your position in the distance. As long as your dog ignores them, you can give your dog praise and a treat. The second you see him fixate on the other dog or show any other sign of aggression (hair standing up, etc.) give your dog a correction by giving a short tug and a firm low toned "NO". It shouldn't take your dog long to realize you will not tolerate the aggression and that if he ignores the other dog, he gets treats. Once this happens you can repeat the training moving the other dog closer until he is no longer trying to lunge at other dogs. You will need to practice this when you and your dog are walking as well.
Another method is BAT training. Read about it here:
Another thing that may help is to find an enclosed area away from home where you can drop the leash and let him drag it around. A helper dog is necessary and it must be a calm well adjusted dog that will ignore your dog's behavior. A labrador tends to be a good choice but any dog that is very calm will work initially. Have your dog wear the basket muzzle and have both dogs in the enclosed area. I would start this after he has learned not to lunge or react on leash. I know you can stop it now, but we want it to not even start before you do this exercise.
Let the dogs both trail their leashes so you can grab either if a fight should occur. If you can muzzle the other dog that will eliminate any chance of injury to your dog as well. Then just let them interact. Since neither dog can hurt the other, he will eventually get tired of the unwanted behavior especially if you engage him in some obedience training or play some games with him. When he is reacting in an acceptable behavior, reward him with calm praise and high value treats. A large number of different dogs will help with this portion as well. Allow him to have time with each to learn that not all dogs are out to dominate or attack him. Hopefully classes will have helped a bit as well with socialization before you get to this point. He just needs to learn different ways of interacting with dogs.
These dogs also need a lot of exercise and stimulating activities. If you have toys for him, be sure to rotate them weekly. Up his obedience training and other training such as tracking or even agility work to help keep him stimulated. These often go a long way toward helping eliminate behavior issues. Walks are very important and you might consider a backpack and put some small cans in the backpack to help tire him out a little easier. A tired dog is less likely to react to other stimulus such as other dogs.
I would seriously consider secondary forms of containment to ensure he does not get out inadvertently. Underground fence system or even instant fence systems work well for an additional barrier to prevent escape. Chicken wire can be buried around the perimeter for dogs that dig to escape. You can also add a top section to walls and fences that angles in a bit to prevent dogs that climb or jump over fences, but the underground and instant fences work very will for these situation.
I hope this information is helpful to you and you are satisfied with my response. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have .