Okay, bear with me because this will be long, LOL!
First off, NO out of control dogs should be allowed anywhere near your guy. You have to make sure all encounters are with calm, friendly dogs with good social skills, because a crazy dog (like you said) will only make things worse.
Is there a particular situation or distance where he is set off? Like, is it only when dogs are approaching and passing by on the sidewalk, or is it just seeing a dog across the street or park that gets him riled up?
You're pretty much going to need to start from the beginning, and avoid any direct nose-to-nose encounters with any dogs for now. You want to find the set point between him being okay and starting to be fearful/aggressive, and step back just a bit from there. Have someone you know walk their dog within sight of him, but just far away enough that he doesn't mind. Give him lots of praise and treats for being good.
After doing this a few times, decrease the distance by a few feet and repeat the treats/praise if he's good. Continue doing this until he is comfortable being close around other dogs who are just passing by (still no meeting dogs just yet). Anytime he starts to react, take a step back and start over a bit farther away, because you moved too fast.
It will take time, but it can be done. Your guy doesn't sound too bad. Once he's comfortable having calm dogs walking past you, then you can try to start calm, positive interactions with the other dog. It would be best if you use the same person and dog this whole time, so he becomes accustomed to that dog while you were desensitizing him.
Sometimes it helps to keep yourself between your dog and the other one, because it helps remind your dog that you are in charge of the situation and that you'll protect him. Walk the dogs side by side, not allowing any direct sniffing, or head-to-head meetings. This helps them realize that this is a friend, and not a foe. You, your dog, your friend, and their dog, become a pack in the dog's mind when you all walk together.
If that goes well, then you can allow your dog to sniff the other dog's behind first (it lets him be dominant and get to know the dog in a less threatening way than face-to-face). Then let the other dog sniff. Keep LOOSE leashes all the time, and be sure YOU are calm, too. Watch for any aggressive/negative posturing, such as squaring off, staring, hackling, etc. If anything like that starts, calmly walk away with your dog to let him cool down, then start the group walk again.
I can't stress enough that you must use a calm, submissive dog that has good social skills for these meetings. Often, a reactive dog seeing that the other dog is just calmly looking at him like, "Dude, what's your deal?" will actually calm down, realizing he's acting like an idiot. You definitely want to stay away from a dog that will react back if your dog acts up. Keep these positive, just-sniffing meetings short and sweet. Repeat them several times, until he is totally relaxed (or even eager) when seeing his new "friend." After that, you can carefully let them off leash together and see how they play. Keep a squirt bottle or hose nearby just in case.
Dogs are naturally social, so once he realizes that this dog is a friend and gets to really play like a dog, he'll see that he's been missing out, and will probably have no problems meeting other friendly, calm dogs. You want to steer clear of hyper, reactive dogs, no matter how good your dog becomes, because you don't want to undo all that work.
Hope this helps, good luck! I know many others on this board have a lot of experience, so I know someone else will chime in.
