In the first sentence, you switched from "mother" to "his". It should have been "her".
For your blank, you could use "natural" or "normal".
Student or Learner
The mother told me that she drank a lot of soy milk, so his boy's skin is very smooth.
I doubted of it, baby skin smooth is naturally.
"Babies born with smooth skin, that is ___"
How to fortify that it is "of course" born with smooth skin?
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.