Omgz! He's so handsome! I love sable GSD's, so much, and his coloring is niiice. =] It is awesome to see such a well-built working bred German Shepherd!
I don't think he looks thin. He is still a pup, and I know my BC was thinner than Biz when he was that age! My BC is about the same proportion-wise now at 5 years, it varies between dogs. I think his tuck looks perfect and it looks like he has good muscle tone.