It sounds like he has a strong prey drive and is dog-aggressive. If your dog is in full-chase, full fight mode, you are not going to be able to call him off. If you would have continued with obedience and training, especially around other dogs, and with distractions from the time he was under two, well maybe you would have a strong enough bond and training that you would be able to call the dog off.
With other dogs, dog-fighting, you need to be aware and react before your dog does. With critters, like squirrels and such, it is possible that an e-collar can give your dog a warning and snap him out of it when he is fixated on the wild critter. Might work, might not. But you will still need to train him with the collar. There really are no short cuts. It is possible that an e-collar could increase the dog's aggressive episode when fighting with another dog.
With dog aggressiveness, and he has already attacked another dog, so this is not simple, I'm afraid of you so stay away type aggression. This is I DON'T WANT YOU HERE aggression. Which means that I don't think getting him used to other dogs from a distance is going to do much good. That will help with dogs who are afraid and barking and lunging to get the other dog to go away. You can work at a distance until he is more comfortable, and then get closer until he gets more comfortable.
I think you will need to correct the aggressive behavior the moment it happens. Which means you need to be hyper-vigilent and watch your dog, and the moment he starts to agress toward another dog, he will need a strong correction. There may be a way to deal with dog-aggression more positively, I don't know.
I manage the situation. I train myself to be aware of my surroundings. Before the dog starts to aggress, I say LEAVE IT! and keep on walking. I do not give the dog the opportunity to decide whether or not to listen and comply. Another thing you can do is use a muzzle. Get him used to the muzzle, and when you take him out put the muzzle on him. Then if you drop the leash, he might chase, but he won't be able to bite.
I would not allow my dogs to go after squirrels or other wildlife, ever.
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