You said the vet said he is healthy-- did they say he was underweight? Can you see his ribs? Weight charts go by average, and the weight can vary by the dog's size and build so just because he weighs less than the chart says, that alone does not necessarily mean he is underweight. You want to be careful, it is better for a puppy to be on the slim side than to be overweight, excess weight is not good for a growing dog.
Personally I prefer the grain free foods, and these also have higher calories incidentally. I would recommend a "large breed puppy" food not just a regular puppy food. Large breed puppy foods are made for larger breeds like German Shepherds which have a lot of growing to do and have different needs than puppies of smaller breeds.
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