My brother developed male breasts, he is just 20 years old. he is also not having hair on his chest. what could be the reason?

Natural: This may occur naturally and related to genetic tendencies but is often associated within being overweight. If your brother is thin, it may require surgery to treat. Prior to that, he should probably be evaluated by his physician to exclude possible hormonal irregularities. Certain drugs and medicatons, particularly marijuana, may be contributing influences.

Gynecomastia: Gynecomastia comes from the greek meaning "gyne" meaning woman and  "mastos"  meaning breasts.  it refers to abnormal development of large mammary glands in males. Although there may be many “causes”, most cases are idiopathic (no specific cause). The use of certain medications, drugs, organ dysfunction (kidney or liver), hormonal imbalances etc. May also be causative.

Anything else?: As brothers, you may be close enough to know whether his sexuality is normal -- is he having powerful erections and ejaculations? If not, it's likely he has XXY or a serious endocrine disorder. A competent physician needs to work him up.