What you are describing is probably gynecomastia. Gynecomastia is a common condition affecting between 40 and 60% of all males. Breast development easily happens during adolescence and may have many possible causes. It can appear as a small lump just under the areola (colored skin around the nipple). During growth, this mass can be tender. Gynecomastia is usually secondary to the normal hormonal imbalances between testosterone and estrogen that commonly occurs during puberty (pubertal gynecomastia) and it may affect up to 40% of adolescent boys during puberty, usually by the age of 14. In these children, the breast tissue is usually less than 4 centimeters in diameter and will disappear without treatment in two years in 75% of children and within 3 years in 90% of children. Sometimes, treatment may be required if he has very large breast development, if it is not regressing, or if it is causing a lot of stress or anxiety in your child. Male breast growth that has not stopped by three years, may not go away. In these situations, an evaluation by an experienced Plastic Surgeon can be helpful, either to provide further reassurance or to surgically remove the breast tissue. Surgical sculpture in skilled hands can be a simple solution for this problem.

Gynecomastia can also be a side effect of some medications and drugs. The most common medication that causes breast development is estrogen, and its effects can be seen in a child who ingests estrogen pills, or even if a family member is using an estrogen cream and has prolonged contact with him. Other medications can include certain antidepressants, drugs used to treat high blood pressure and tuberculosis, and some chemotherapy agents. Street drugs that may cause gynecomastia include marijuana, heroin and alcohol. Anabolic steroids can also stimulate male breast growth.

There are also some diseases that can cause gynecomastia, including certain endocrine disorders, tumors and chronic diseases. Klinefelter's Syndrome is one such problem where there is an extra chromosome that can impair testosterone production and development of male characteristics (more information). Early recognition and appropriate treatment can be very important.
http://www.keepkidshealthy.com/adolescent/adolescentproblems/gynecomastia.html

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Regards

Dr. Gupta