When I first took Blitzen in for shots, he was about 12 weeks and weighed in at 15 pounds. We were worried about his weight and the vet said that he did look a little skinny, although doberman puppies will always be very lean. The vet was worried it might be worms so he did a fecal exam, etc, but Blitz was clear. At the time he was on Blue Buffalo Lamb and Rice adult food which my breeder had the puppies on originally. My vet recommended that we continue to feed him the 1 cup a meal 3 times a day as we had been, but to add either a low sodium chicken broth or some cottage cheese to his bowl and mix it around to encourage him to eat (at the time we would feed him and he would eat a few pieces and literally not touch the bowl again... regardless of whether he had eaten much of his previous meal or not... it was scary!)
The boyfriend and I spent about a month practically spoon feeding the little guy using chicken broth, cottage cheese, peanut butter... basically the works in order to get him to actually eat some of his food. It was really stressful and frustrating however just a few weeks and he's now 25 pounds and looking much better so it was definitely worth it.
We started changing his food over to NutroMax, which doesn't seem as quality as Buffalo but Blitzen will eat it without any encouragement so looks like we're sticking with it!
Anyway, low sodium chicken broth, cottage cheese, and peanut butter worked for me.