What you want your dog to learn is that these people are no threat and don't matter. I like to use desensitization for this, mainly because you can do it without help. Take your dog to a park, staying back away from any trails or hiking paths far enough that your dog is able to stay calm, and let him just observe people from that comfortable distance. Work on obedience, play with him, dogs typically won't be very engaged at either of these activities if they're feeling stressed. So once he does get comfortable enough to do them, you know he's adjusted his thinking and is relatively calm! From there incrementally move a bit closer and repeat.
It takes time and patience, and it's not something you can rush. But again, it's an easy way to work on the issue without help from others. Once he's a bit more calm around people, take him to home depot or tractor supply and just walk him around the parking lot. There's usually lots of car and pedestrian traffic, so it's a great way to further desensitize him! The main issue I had with my dog in doing this was keeping people away. But it's important to do so. No greetings, no pets, and no treats. Just calmly walking around and ignoring people as the background you want your dog to see them as.
It's not the size of the dog in the fight, it's the size of the fight in the dog. Mark Twain