It's so hard with puppies, because they go through growth spurts where they shoot up and then get a bit skinny. He's certainly not so thin I'd worry. You might up his food just a little, and as long as he doesn't get loose stool, or start looking too pudgy, it would be fine.
See we were doing 2 cups three times a day and he was having frequent loose stools and I read here that could be from over feeding, so we cut back. But now I think he looks too thin. Stool is now solid and 3-4 times a day.
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