Just breastfeed the baby. Breastmilk is liquid gold and it has all the essential nutrients required by a baby.

Mother's milk is most important for infants. Give it exclusively as much as possible. This solely helps in building immunity. If its not possible due to any reason then you can opt for fm. Aptamil will be a good choice but do consult the paediatric first.

Try to breast feed your baby , mother's milk is sufficient for infants and provides all essential nutrients which helps to develop immunity in kids.