i replied to your previous post will also reply to this the same and add something.
i have had several male newfoundlands that were this thin thru the growth stage, they were healthy, no loose poops, just how they grew, i would much rather they be thin, than overweight, esp for our breed.
now, on to the issue of diet, i mentioned that pork formula back to basic food is doing huge favors for GSD and Boxers with irritable bowel syndrome.
you might want to try that.
i had a GSD with Pancreatitis that was a rack of bones when she went thru her heat cycles, my parents dog, the vet wouldnt spay her thinking her to high risk, and the neighbors said the same thing to me, i went up to them, and politely explained, that, she has pancreatitis wich affects their weight and is why she is skinny, and, they were actually embarassed they accused me.
so maybe you could go and tell him you are dealing with health issues next time so it does not eat away at you and just makes you even more frustrated.