Genetics. Genotype. Can you explain this,below, to me

My little brother was born with blue eyes but they changed to brown. In science we are learning about genetics. His mom has brown eyes and his dad has blue. If brown is dominant and blue recessive how did the recessive color mask the dominant color when he was born? If he had the Genotype bb how did it change to Bb?

The pigment that makes the eyes brown may not have been deposited into the iris by the time he was born. Many babies are born with blue eyes and later have a different eye color. His genotype has not changed.

Blue eyes have no melanin, the brown pigment which produces brown coloured eyes and many babies are born with blue eyes which change as melanin synthesis starts. It is quite variable though.

Although you will learn about inheritance of eye colour as being straightforward with two alleles, in reality it is quite complex with several genes involved.

Learn your basic genetics, but be aware that sometimes complex pictures are simplified to help beginners understand it.