i have the same experienced when my baby was 1 week old, my dad said formula is much better than breastfeed. (old-fashioned mind)
you know what I do, I brought him to go together with me when my son has regular check-up appointment with doc and let her explain right in front of him how breastfeed good for the infant, then he won't say anything negative thing to me & cook me some soup with papaya & fishes to express his supportive.