I really feel for your son - puberty is bad enough without such an obvious
problem in the locker room!  You probably know already, but 60-70% of male
teens experience breast enlargement (gynecomastia) between 12 and 15 years of
age. He is older than the usual age for this, but not too far out of the
norm.

Breast tissue grows whenever the ratio of the body's estrogen is increased
relative to the body's testosterone. Although estrogen is thought of as the
female hormone and testosterone the male hormone, males and females have both in differing amounts.

Early in a boy's pubertal development, he has not yet experienced much
testosterone secretion. Because he has relatively more estrogen present in his
body, one or both breasts may be stimulated to enlarge. The breast
enlargement usually disappears spontaneously within 6-24 months when his
male hormone overtakes his estrogen.  So again, he is still within an
expected norm for resolution.

Unfortunately, there is no way to decrease the swelling without manipulating
his hormones or doing surgery. Neither is a good option.  Manipulating
hormones during puberty can cause undesirable and long term side effects and
surgery is only warranted if it does not resolve all by itself.  However, I
do hope that someone has considered other reasons for this problem.  Of
course, it may just be a 'common part of puberty' but if there is a
treatable cause - you would want to know so you could deal with it.  A few
considerations to rule out are endocrine tumors (preventing the normal
testosterone development), thyroid disease, or marijuana use.

If I were you I would make sure that all abnormal causes of this were ruled
out and then you are left with supporting him through the painful
transitions of puberty.  No easy task, but well worth it.

Best of luck to you,

Dr. Kerr