Unusual growth?

[Slvohsen]

I bought Blitz at 8 weeks from what I thought was a reputable breeder. His parents were healthy, OFA checked, fed holistic food and Recieved proper socialization and health care. His mom is German lines and his father American. However he was small. Only 7 pounds. I soon found out he is EXTREMELY picky. Within days of bringing him home he refused to eat, even the food he was on (Taste of the Wild). Desperate to get him on food I bought a few brands that I liked and he finally began eating Bil-Jac large breed, as it was the only one he chose off a plate of about 5 dry foods. Soon however he was off it again so I tried Royal Canin. He ate that but came off within a week. By this time I knew he had to be the runt. He was not growing and was thin. I made satin balls just to make sure he ate and he loved these but I did not want to pursue them as long term choices. I work at a vet clinic and he is weighed each week but is growing so slow compared to my older one and most pups in general. At 13 weeks he only weighed 17 pounds. He is finally eating Bil-Jac again but I have to mix it with rice and he still has no issues skipping a meal or two. I've never had issues with a puppy like this. He is SOOOO picky and I don't want to change his food again for fear he'll come off. He is on Nuvet, 1000 mg of Vitamin C, 500 mg of Fish oil and GNC coporphagy pills. I try my hardest to make him healthy but worry about his size. He plays normally and his ears have been up since the day I got him. He's normal in all other aspects and is extremely smart even for a GSD (I work at a dog kennel too with multiple GSD and my trainer even says he is extremely smart) but the size and weight is a big deal. On a side note his testicals were already descended and mostly dropped at 8 weeks which both of my vets were surprised at. I would think the testosterone would make him growing even faster but it's not. He has no health issues except dry skin and now is batteling and intestinal bacterial infection but this has only occurred in the past 4 days. Has anyone dealt with a pup similar to mine? It concerns me as I've waited years for this pup and I adore him and want the best for him.