My son who is a preemie isn't gaining any weight

He was born July 14th and he weight 4lbs 13oz 17 1/2 long. His follow-up doctor appointment was last wednesday and he weight 4lbs 6oz. The nurses in the hospital acted like it was normal for him to lose weight since he is a month early due to past vaginal surgeries i had in the past that kept me from going full term. I feed him Enfamil premium every 3 hours but his follow up doctor said last week to feed him every hour and a half. I am just wondering if all of this is normal since he is a preemie? He goes to his pediatrician friday and hoping she will tell us more to do. This is my first newborn so I am trying to learn through all of your moms out there.