My 13 week old German Shepherd is suddenly reluctant to go on leashed walks, what should I do?

It's normal, just work your puppy engagement and relationship building and avoid putting him in situations that make him worried for now. I don't usually take young puppies on leashed walks around the neighborhood much anyway due to all the things that "might" frighten a puppy (a loose dog charging, a big garbage truck making a noise, a clueless human making the puppy feel uncomfortable)

Until I have a solid relationship and training communication system in place I don't take puppies into environments that I can't control very often. I hope this makes sense.