Not true protection from a 15 week old.
Instead, it's the normal FEAR stage that our pups go thru. We have to help them and deal with the fear, not accidentally encourage it by thinking our pup has naturally and brilliantly started protecting the family.
Truthfully, look at what the puppy was 'protecting' your son from. A normal, happy, totally NON-threatening neighbor.
You should have initially IGNORED your poor afraid puppy and gone ahead of them to greet your neighbor. As the family leader, your pup then should have immediately quieted and started thinking 'I was a dope, look they are ok' and then the puppy should have then followed.
If you had your training bag of treats, even better, could have handed some to the neighbor who could have fed the pup when it went nearer.
Hopefully, you've been driving the puppy to all your friends/neighbors/grocery store/main st USA so you can continue to reinforce that when you are calm and friendly, the puppy can calm. Mom's in charge, mom will deal with the situation, look to and learn from MOM!
Great time to find and start up puppy/dog classes!
Here's info about socialization and fear stages (and how we much this up all the time!)
Von Stroman German Shepherd Puppy Primer, Part I
Von Falconer K-9 Puppies 1st Night To 1st Year