Hi, Good day. Pubertal gynecomastia is common thing. This occurs in young boys during puberty. They have a surge of hormone called Testosterone which is then converted in to estrogen. Estrogen is responsible for breast growth. Gynecomastia can develop in a single breast and then develop in other breast as well. Usually it disappears in months to years. However, physical examination is needed to decide if it is pure gynecomastia /fat or any other issues. There are number of other swellings in breast area other than true gland development ( gynecomastia). But as this swelling is just under the nipple, it is likely to be true breast gland . Testes examination and assessment of SMR ( sexual maturity rating) also is important. The pediatrician/endocrinologist may suggest few tests based on examination or opt to observe if there is nothing suspicious in physical examination. Please take him to doctor and let him decide. Regards Binu