this is tragic.
i hope your son recovers quickly w/o too much scarring, physical and emotionally... and that the rest of u recover from the shock as well... it is a terrible thing.
one thing i know for sure, and learned the hard way the importance to follow the rule **never leave a small child alone with a dog. i mean, we hear it everywhere.... esp a newer dog (to the family) and still we leave them alone just for a minute. *i did this one evening... regretfully. my son who is for the most part quite gentle and sweet-- and minds very good... well he is a young child and they can be unpredictable. just like dogs.
so yes, he got nipped. it was all dealt with. and we still have the dog, for many reasons, like the dog had tail injury at time, and b/c my son completely provoked it, (pulling on tail!) and dog gave warning growls and snapped at air, before finally nipping hand holding tail... and b/c of dogs behavior after, and really the reason it happened is b/c i should not have left room knowing my son was bothering the dog... but yah, i thought my good son was minding me as i stepped out for just a second. ((the dog has never showed any other type of aggression towards my son or any actual person))
ok, so that isn't meant to be blaming. but sometimes there isn't a clear answer. there is only prevention. so for future now, like me, you will not leave them alone either.
but also, since u are asking why, i admit only read thru the first several pgs, so maybe this has been sugg? if so, my apologies.
but, and i am just wondering aloud here- is it possible your son is special needs? u haven't said. and most likely that means he isn't. BUT i have heard of dogs, not nec gsds, but dogs in general--- showing aggression towards kids w/ special needs, and by special needs i mean just that ie autism etc and YES i have also heard amazing stories (mostly amzing vs bad stories) of how dogs treat kids w/ special needs.
but back to your situation, it was just a thought. plz don't read anything into aside from what it is.
i have heard of dogs who freaked out when a person had a seizure. then u hear about the dogs who can detect onset of a seizure. so there are both sides. again, not saying your son has anything going on....
moving past that possibility.... b/c i too have a young child. well, they make a LOT of crazy high pitched noises in play! boys esp! mine sure does. or did. he had a friend over the other day (who by the way is autistic and who cares about that, we love this kid) and this kid was making all these high pitched sounds. my dog was really flipping out. mostly his head was moving side tilt to side tilt kinda like "wtf is that crazy sound coming from a human?" at the boy.... the boy wouldn't stop the noise and i felt compelled to remove the dogs. just b/c i wasn't sure what might happen. the dog was very confused. it was almost bird tweeting whimpering squeaky sounds.
a trainer i ment'd this to told me that just leashing my dog to ensure safety might have been smarter as my son will prob cont to play with this boy, which means the odd noises will also be around. so better to desensitize my dog to it or something....
anyhow.
i am truly sry this has happened to your baby and again, well wishes
plz let us know how your son is doing...