So glad you took the research time and found out what *not* to feed your dog
I thought I had a major health problem w/my own dog(s) early on. My now 18mos old male would skip meals (just not eat them) sometimes both meals for a day or two at a time. Being new to big dogs this was literally freaking me out. My Dad always had dogs and they always seemed to eat just fine. People and the vet commented he was to thin. I literally panicked some more and tried everything under the sun I could think of. There were days I was actually in sobbing tears thinking I was starving my dog to death and what could I do to fix this kind of thing. I had *never* known a dog not to eat.
He was getting Canadae - a high quality food but I found out that even though it's great food it supposedly has been known to not help some GSDs put on or hold their weight. Some foods no matter how good they are just don't agree w/a dog's metabolism or something like that. So, we tried another food and bingo! He started eating better and put on weight.
The vet said that reality was he's not "food driven" which to her means I'll never have a fat dog
hopefully she's right. If he's not hungry he just doesn't eat. He may skip a meal, may skip a full day's meals and even into the second day. Then he turns right around and goes back to eating normally. About once or twice a month he will finish his meal and sit by his bowl wanting more. Those times, I will feed him until he walks away. He knows when he's full and that's it, he's done. Being that his usual consumption per meal when he does eat is well within the guidelines I do not worry about that once or twice a month when he's extra hungry. I chalk it up to the same thing happens w/people.
I have an 8mos old female who is now exhibiting the same eating behaviors as my male. I feed her now the same amount as I feed my male. 2C dry http://www.eaglepack.com/Pages/HS_Lamb.html
mixed with one slice bread broken into pieces for each dog and 1/2 can Merrick's http://www.merrickpetcare.com/
(they share a large can each meal). What they don't eat if there's enough to bother saving, I put in a covered container in the fridge for a later meal.
Why the bread? Well, fiber is good for them according to my vet. I found this out when my male ate a (nightlight) light bulb. In order to help this pass, the vet suggested fiber (bread or pumpkin) mixed with half & half creamer mixed in his food. During the few days afterwards I found that my backyard was MUCH easier to pick up <eg> and so I just continue with the bread each meal. On top of that because both have gone through a "thin" period the bread helped them put on weight as well. A slice of oat or wheat bread in your dog's food may help with the runs. I should think it would unless there's a medical issue you're not aware of. Or you can add canned pumpkin, or chunks of pumpkin.
My female started out 4mos or so ago when we got her at about 1 3/4C dry and the rest of the mixture. She eventually started eating more a month or so ago so now gets the same 2C my male does.
I don't know what she weighs now because the last time she was weighed was in October when she was spayed. She was 53 or 56lbs then. I want to say both of them were 60-65lbs at about 9mos of age but don't quote me. They were both considered to be "thin looking" at that age. As my male got older he filled out very nicely and is no longer considered thin. Could be yours is having a growth spurt where he's all arms, legs and body length but not enough weight and he'll catch up over the next few months.
You really can't judge the dog's weight by how much other dogs of the same breed or age weigh. For example my daughter has a female GSD who's one month older than my female. Her GSD is *very* small in comparison to both of mine. To see the 3 together she's (my own term) like a little dwarf compared to mine. I don't think there's a single thing wrong with her, I just think she's going to be a smaller GSD when full grown. Or ... I suppose she could have a growth spurt yet and catch up though.
My own female's parents are quite large dogs. Daddy is like 130lbs and Mom is probably 90'ish. We do expect our female to be taller than our male when full grown. At 8mos she's already a hair taller than he is but she has a more dainty frame. He's 75'ish lbs so not a huge GSD but he's built *very* nicely. If I was a female GSD I'd call him "buff" LOL! I'm told 75lbs is pretty much average. When you get one over 85'ish lbs they're considered large. I have no clue how accurate that is.