Here is another case that John Fisher answered about a germen shepherd who was showing the same aggression towards other dogs that yours is and explains why you should never jerk the lead or use any type of punishment correction methods to treat this.

"Question
My 2 year old german shepherd dog is very aggressive towards all dogs when he is with me. It doesn't matter whether it is a dog or a bitch, a puppy or an adult; as soon as he sees them he flies out at them. If I spot the dog first, I give him a sharp check on his choke chain to bring him to heel, about turn him and walk in the other direction. If he spots it first, he often takes me by surprise and, even though I am a big built man, he has pulled me off my feet on more than a few occasions. I have sent him away for training and when I collected him he was running loose in a compound with four or five other dogs with no trouble at all. The trainer said that the kennel maid used to walk him on the lead through the local town and he was no trouble when they met another dog. None of the dog club instructors who have walked him have ever experienced any problems. My vet has castrated him, but that made no difference at all. Do you think he is being over-protective towards me and, if so, what can I do about it?

Answer
There may be an element of protective behaviour involved and it would be well for you to refer to the rank reversal procedures to ensure that your dog sees you as the protector of the territory. But I am more inclined to think that the way you have been handling the situation has had the greatest influence upon your dog's aggressive behaviour. The fact that it is non-selective aggression (all dogs, regardless of status); the fact that he will run free with other dogs; the fact that he does not behave aggresssively with other people would all point to the fact that he is not an aggressive dog by nature. Therefore, the behaviour must be learned and the common denominator is you.
If you think about it from your dog's point of view, whenever a dog appears on the horizon, you give him a sharp, and possibly painful tug on the chain and head off in the other direction. Now that would not have happened if the other dog had not appeared, therefore it must be the other dog's fault. The presence of another dog has become a prelude to him getting punished. I know what I would do if I were your dog. I would try to chase off the other dog before you saw it. When other people have got hold of his lead, he has not been punished and so there was no need to show aggression.
I can understand the fact that once the dog starts to become aggressive the owner starts to lose confidence and, as a result, starts to overreact. But it is this overreaction that creates a vicious circle and compounds the problem. There are two problems to overcome:

1. To increase your confidence
2. To change the dog's expectations about what the presence of another dog means and therefore reduce the aggression.

This should be done in the following ways:

A) Change the choke chain to a broad leather collar so that there is no pain-associated element involved.
B)Change the short restrictive lead to a strong flexi leash. This will increase your dog's freedom of movement, and that in itself will have a calming effect.
C) Temporarily, fit him with a soft muzzle (mikki muzzle are ideal and obtainable from most petshops). This is to ensure that there can be no damage if he does take you by surprise, but it is primarily designed to increase your confidence. This type of muzzle is designed to restrict the movement of the dog's jaw, making biting impossible but allowing you to feed the dog, which is the next stage. (Do not leave it on too long in hot weather!)
D) If he usually gets three walks a day, make sure that you split his daily food ration into three equal portions. These should be further split into three or four smaller portions and put into a plastic bag to be taken with you on a walk. For the next week or so, your dog will only get fed when out walking, with a portion being given after he meets another dog.

The procedure will be as follows. When you see another dog, be prepared for him to lunge out, which he surely will out of habit. Do not apply the brake of the flexi leash until he does, so that you do not transmit your tension to him. Stop him if he does lunge - the collar will ensure that he does not feel any pain - but do not say anything negative. Gently call him back to you and take a portion of food from your pocket. Let him have that portion and continue walking. Repeat the procedure until he has had a third of his daily ration and then go home. Keep a portion handy in case you meet another dog.
Although this may sound as if you will be giving him a reward for lunging out at other dogs, in fact it will have the reverse effect. Because he will not be punished, either physically or verbally, when another dog appears, he will begin to view other dogs as a prelude to being fed. When you reach the stage when he looks at you when another dog appears, you can despense with the muzzle. Quite soon, the food reward can be replaced with the occasional tit-bit, or a game with his favourite toy, or just some verbal praise."

Hope this question and answer helps you in overcoming your own dog's aggression towards other dogs.
Use info in the above post to also address any rank issues that may exist.