Baby feeding

My baby is 1 month old now. Used to follow feeding schedule every 3 hrs but now he keeps crying and we do not have a fixed schedule now. He is on fm and drinks ranging from 1hr, 1.5hrs, 2hr, 2.5hrs, mostly 1.5hrs or 2hr. He is 5.2kg and each feed we feed him 120ml. If timing is too close like 1 hr, we feed him 80ml. Not sure why my baby keep hungry? Is this normal?