Hello and thanks for your question.
It's my first priority to help you.
I understand your concern.
As per your description, such small lump under nipple can be associated with hormone changes/surges with subsequent temporary enlargement of such glandular tissue.
It is very common for boys early in puberty to develop breast tissue under one or both nipples. It's usually mobile and may be slightly tender. Most commonly it's only on one side. The lump will eventually disappear in most cases.
If it is not painful, inflamed, bleeding, or associated with fever or discharge, then this may be monitored for resolution over the next few weeks or examined by the pediatrician.
In the meantime OTC Advil and Allegra can be given for few days.Warm compresses can be helpful if he has tenderness around it.
If it's not having associated symptoms then a watchful wait can be considered.
Feel free for further queries. It's good to discuss rather than quit.
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