New born baby drink formula milk

My baby boy is 6days old, we feed him every 3hours, most of the times he only drink 30-40ml milk, only 1-2 times drink 50ml milk. I am concern and worried about his feeding, wondering is it normal? I intend to prepare the formula milk and keep in vacuum flask, will the nutrition of the milk went off or reduced? Does any mummy encountered this problem before?