Baby was 90% when born, 25% at 1 month and down to 10% in 5th mo

My baby was born in the 90% percentile and is Asian American. She went down to 25% in a month and the doctor said its common for asian american babies. Also, my baby is just breast fed. My family refuses to feed formula even if she is hungry and there is no breast milk. How will it effect her development? Doctor recommended we start cereal but my family doesn't want to give her any solid food for an year. Please advice