Fearful puppy has the opposite problems to most other puppy owners I knows

My little Mudi puppy is almost 5 month old and scared of everything and everyone, including other dogs, even the ones that are smaller than him. He's alright at home and in the garden, but as soon as I take him on a walk, he just wants to run back home. I can't take him to the street because it just ends up being an exercise in straining my arm. When I take him to dog school, he barks at any puppy that gets close to him, and he's wary of most strangers. It just doesn't feel like he's getting any better about it, if anything, he's just getting worse. Just about every time it seems like he's about to be brave, a large van pulls up, or someone makes a sudden move, or a loud noise startles him, and we're back to square one. Even bad script writers would handwave away the amount of times something that would startle a fearful puppy happens at exactly the wrong time as "too contrived."