(photo is Bailey - Lure-walking)
Leash Frustration or ‘Leash Aggression’ is a very aggressive-looking reaction to other dogs (or people) when on-leash. This comes from the feeling of frustration at not being able to freely investigate the other dog (or person). It doesn't necessarily mean that he wants to fight, but he does associate the frustrated feeling with the presence of other dogs (or people), so the aggressive behavior is directed at them. With intense frustration, there can be a loss of self control which can increase the likelihood of a bite, even if your dog is not normally aggressive.
From my perspective, as a behavior consultant and trainer, this is an indication that the dog is confused about how he should be acting. The confusion leads to frustration and manifests in the aggressive reaction: generally lunging, barking, snarling and whining. The key to stopping the aggressive response, is to train the dog what you want him to do when he sees another dog. If he has been practicing this behavior for some time, you may begin by creating a positive association with other dogs by sitting in a "safe" location (one that is far enough away for the dog to not react) and having neutral dogs pass by. Each time a dog comes into view, give LOTS of treats, at first for free and successively, and then for calm behavior with a second or two in between. The next step, if you can arrange it is to have neutral dogs remain stationary and walk your dog at a "safe" distance, frequently giving treats, every two or three steps. If he looks at one of the other dogs, that is fine, mark it ("YES!) and treat him for looking and not reacting, even at this safe distance. Periodically stop and have your dog sit for a few moments.
The next step is to have a neutral dog moving at the same time as you. You can walk parallel to each other and give frequent food rewards for remaining in "heel" position (at your side, attentive to you). You can be as far away from the neutral dog as necessary to achieve success! You will gradually move closer with each pass. You can also walk toward each other with the same process of gradually increasing the difficulty. This process can take several weeks, so don't move too quickly.
Of course, if you don't have access to a neutral dog, you will need to do this in safe locations where you know you will see dogs and try not to get too close. You can sit in your car for the first part and just give him treats when he sees other dogs, maybe in the PetCo parking lot or at a park. On your walks, stop and ask him to sit frequently when there are no dogs around, so when you ask him when there is a dog, he won't be surprised. Try to prevent the reactions by crossing the street or changing direction when you see a dog (or person) come into your path. Remain calm, your anxiety can make the reaction worse, as your dog may feel there is reason to be anxious, or that he needs to defend you. If you feel yourself getting nervous, try quietly singing a nursery rhyme.
Remember success is measured by good behavior or the lack of a reaction, it is NOT a measure of how close you can get before a reaction...avoid the reactions and PRACTICE SUCCESS!