The standard for gsd's call for "aloof",,and while yours may be 'shy', he may just be "aloof" and your thinking he is shy?
How does he act if someone comes up to him? Is he cowering in fear? Hiding behind you? Tries to bite them? Growling?
That^^ is fear..
Aloof is, tolerance but could give a poop about anyone..Not slobbering all over people, again, tolerant, but just not interested..
if it's aloofness, he's correct
They aren't supposed to be social butterflies but should be tolerant and they aren't golden retreivers