Dogs, in general, are ‘pack’ animals. They recognize their own ‘pack’ as this boy is part of his pack, but the strange dog is not, so danger!.................
I think the German Shepherd just saw the kid as a sheep he was supposed to protect from the “wolf”.
About 17 years ago we had a female Irish Setter for the family dog. My son who was about nine at the time was opening the front door to go out on the porch and holler for the dog to come inside for the night. We had an invisible barrier and she ran in the yard and played during the day when we were gone.
When he opened that door the setter came out of no where and knocked him back towards the door and a 400-500lbs black bear emerged from the wood line heading for our porch. My son came in screaming a bear is going to kill Katie a bears going to kill Katie! I went to the door along with my wife and sure enough Katie and the bear had squared up about ten feet apart and both were not yielding any ground and both growling and going crazy, the bear roaring and popping its teeth, Katie was in full attack mode.
I told my wife, hand me the shotgun in the front closet and she said what are you going to do! I said when I shoot any guns Katie likes to kennel up, I’m going to fire into the ground, if she runs up on the porch grab her and both of you get inside. I fired and sure enough Katie broke and ran for the porch and my wife grabbed her collar and they went inside and the bear stopped and looked at me. I racked another round in the chamber and fired again into the ground and he slowly turned and ambled off into the woods. Katie lived 12 years and was a good protector of our family.