Anyone with a preemie please help??!!?

Okay so here it goes when i was 30 weeks pregnant i was diagnosed with pre-clamsyia at 33 weeks it got severe they tole me they were going to have to take him out so they gave me 2 steroid shots and at 33 weeks 5 days i under went an emergency c-section i was worried but when i heard my son crying and screaming to the top of his lungs i knew he would be okay. he was born 3lbs 12oz and in the nicu he spent 2 weeks he left the hospital with 4lbs 1oz and 2 days later we took him to go see his pediatrician and he was weighing 4lbs 10oz. its been 2weeks since then and his next apt is on wednesday and i know hes at 5lbs something. He seems to be doing just like a term baby he would of been full term yesterday at 37 weeks. however i get scared because hes drinking 2 oz every 3 hours and i feel like sometime he leave a couple of mils and he sometimes gets constipated his doctor says its of no concern maybe im just worried but is this normal??