My son 13.has a hard ball behind his right nipple. painful if pressed. what should i do? thank you.johan

Gynecomastia: More likely than not, the hard ball behind his nipple represents breast tissue responding to an excess of estrogen relative to testosterone, possibly due to being overweight. Of course, i can't be absolutely sure so he definitely needs to be evaluated by his family physician or pediatrician. Sometimes, medications can upset balance between estrogen & testosterone. Other times, it's just time.

Breast bud: I understand you to say that your sonia 13 years. Many boys during puberty may experience temporary breast growth from the hormones in his body. This is temporary and will go away. But to be sure it is nothing else, please see your doctor.

Male breast tissue: Breast buds affect ~1/2 males in early puberty and may affect one or both sides concurrently. The process reflects an awakening of the body to hormonal changes. It generally persists for a year or less & disappears gradually. Rare cases can become prominent. Discuss it at any regular dr visit, but i wouldn't go in for this alone unless your kid is going bonkers over the issue & needs reassurance.