Lead aggression is directly related to the dog’s perception of other dogs. Whether it is a learned bad habit or a result of a traumatic experience, aggressive or overly excited behaviour is a reaction to the presence of other dogs. Hence, you will have to change your dog’s associations with other dogs and to teach him an alternative behaviour.
If your dog is scared of other dog, your best bet would be to teach him that other dogs mean good things. In other words, you will need to make a new association in your dog’s mind. To do so, you will need really good treats that will occupy your dog for a minute or so. Carrots or doggy chewing sticks are good examples.
Always begin training in your dog’s comfort zone. You will need to figure out what distance to other dogs is still comfortable for your dog. Have your treats ready. Try to catch the moment when your dog has just noticed the other dog. At this moment give your dog a treat, wait for him to finish it and walk away – calmly but quickly. Repeat this scenario until your dog begins to look at you after he notices other dogs. When your dog will have a reliable connection between other dogs and treats coming from you, you can move decrease the distance to other dogs.
During this initial training phase your goal is to help your dog to avoid failure. It can be a bit tricky, because you will meet other dogs all the time during walks. You may want to shift your walking schedule a little to avoid dog walking “rush hours” or to choose less popular routes. If you see a dog that is likely to cross your path, turn around and walk away, ideally, before your dog goes mad. Even if you cannot escape the other dog, make sure that you ignore inappropriate behaviour. Don’t shout, don’t jerk the lead, and don’t show any emotions. Just hold the lead firmly and lead your dog away from the scene.
Eventually, your dog will learn that other dogs mean good things are coming. You know that it happened, if your dog turns to you after noticing a dog across the street. Make sure that you always have plenty of treats with you during walks. If you ran out of treats, you can always praise your dog or reward him with a play or a short run right afterwards.
All in all, this is a simple technique that will help you to manage lead aggression/reaction. Usually you can see the results quite soon, if you use it consistently, meaning, during each walk. Good luck!