Reactivity

What you’ll want to do is start exposing him to “those DOGS!!” at a level where he’s comfy, and has little to no response to the other dog.  Keep him happy and occupied by pairing the experience with things he likes (FOOD or toys). As you increase how close you get to the other dog while keeping him happy and
occupied, he’ll start to associate the other dogs with happier thoughts.

Most Important Points:

Stay below threshold - no barking, growling, or otherwise major reaction. 

Treats must come at a rapid pace, the entire time.  They should come at a rapid enough rate that he doesn’t have time in between to decide that that dog is still evil. You can eventually slow it down, but initially….keep them coming, as long as the trigger is visible!

Get him to perform replacement behaviors that are incompatible with the reactive behavior. Make sure you pick things that he enjoys so that he’s making that positive association.  Get/train him to focus on you and do other fun things that will keep his attention on you.  

Always start at a distance (whenever possible) where you can keep him happy and focused on you, keep his attention on you the entire time, then end on a happy note, before he gets tired, and before you run out of treats.  You will, along the way, get surprised by another dog.  He’ll probably have an outburst.  When this happens, just remove him as quickly as possible without trying to give him any commands – just turn and happily run the other direction.  When he gets to a distance that he can settle down, stop, and work on some 

The Breakdown...  So you basically just go on your normal walks, and other “training adventures” (like to the park where you have plenty of space to reward him as other dogs walk by) and whenever another dog goes by, reward
(appropriate behavior) with treats (be a pez dispenser), and have him pay attention to you instead of worrying about the other dog, and at the same time, learning that good things come when dogs are around. The better your technique, timing, and ability to use your body movement to help keep him focused on you, the more successful and efficient you’ll be. If you have trouble keeping his focus away from the other dog, it’s likely that there isn’t enough distance, so give him more space, or completely remove him from the situation, play some fun games with him, and end the session (always remembering to end things on a happy, positive note).