A lump under or near the breast nipple and breast soreness in male is known as gynecomastia.
When this condition appears at the onset of puberty is known as pubertal gynecomastia and usually affects boys aged 10-12 years.
This is benign condition, results from a transient increase in estradiol concentration at the onset of puberty in boys, and generally regresses within 18 months.
Sometimes, there can be an underlying condition like kidney dysfunction, liver diseases, hormonal imbalance like hyperthroidism, testicular tumor, bad nutrition, or it can be a side effect of using certain medications and recreational drugs like alcohol, marijuana, and heroin.
It will be a good idea to be checked by his physician for complete physical examination, lab tests like liver tests, urine tests, hormonal status test, and you may also consult a endocrinologist (doctor for hormones) and/or toracal (chest) surgeon who is specialized in breast diseases.
My personal opinion is that this is just a benign pubertal gynecomastia.