Thank you for posting your query
Let me start by presenting generally the various conditions that could point out as cause for your son's painful lump of his left breast. Normally in order to give a proper guess of what your son can have will require a physical examination and other imagery and laboratory studies (including a biopsy)
The different most probably causes (given your son's age and according to probability of occurrence for him), we have:
- Chronic infection/mastitis (which was not treated at the onset hence has become a chronic inflammation). If the case, then appropriate antibiotics probably after analysis of the biopsy and culture.
- Benign mass such as cyst, fibroadenoma
- inflammatory breast cancer
(less likely because of his age, but very possible)
- Gynecomastia: is possible but less likely due to the fact that it is painful. Not usually painful.
Other causes will need to be determined after biopsy and imagery (CT scan, mammography
or MRI etc).
You can pay a visit to an oncologist
or cancer specialist or pediatrician
Hope this helps