Like I mentioned earlier, Rare 2-3 month old gets any allergies from any food cause they’re bodies still have mommy’s milk so they are well protected. Like a human baby drinking Mom’s breast milk. Good immune system. That being said, if his parents were fed only RC that’s not healthy for the pups. You mentioned he is getting scabs all over his back and has a very running nose with thick green boogers. Did the vet do a scrap to analyze the skin issue? Also as you probably know, thick green boogers is a sign of infection.