If you are free feeding stop it. Measure out the food and offer it three times a day. Pick the bowl up after 15 minutes if he isn't eating and put down at the next scheduled time. If you are worried then he can have lots of scheduled times, at least 2 hours apart though to give him a chance to get hungry.
I am not crazy about RC nor kibble in general but that is just me. Look for a large breed puppy food with more named meats at the top of the ingredient list and fewer grains. That corn gluten meal is cheap hard to digest 60% protein, a whole lot of the stated protein comes from it. Animal proteins are much easier for dogs to digest. Look for low calcium/phosphorus level/ratio. Calcium should be 1.2 to P. Lots of foods are much higher than that. Protein and fat are fine if high, it is the minerals that mess up large breed puppies growth. If you decide on a new feed then switch it slowly, 1/4 new at first. Loose stool is fine, frequent squirts not so good. GSD can have finicky systems so go slow.
Also the bag's feeding instructions can be high. Feed the dog. Ribby feed more, fluffy feed less. Puppies might eat less when teething, more when having a growth spurt.