I just thought I'd add a bit of reassurance to this issue. It is very common to have some breast tissue develop in pubertal boys. This is because both men and women actually have breast tissue. During puberty, levels of testosterone and estrogen rise in boys (and girls). This rise can cause some initial enlargement of the breast tissue that subsequently resolves as the level of testosterone increases and the estrogen levels settle down a bit in boys, while in girls the estrogen levels continue to increase inducing further development of breast tissue.
Breast cancer, while possible, would be incredibly rare at this age. It would NOT generally be advised to have a biopsy of this tissue in a pubertal aged child. Instead, reassurance and continued observation are usually the appropriate choice.
Advice and opinions given are based on the information provided and must not be considered an official medical recommendation. The limitations of the internet prevent an appropriate evaluation. Always consult your doctor if there is concern.