Gynecomastia is the development of abnormally large breasts in men. Before I go any further, let me ask if your son is overweight. If he is, that could explain the breasts. If he is thin or average size, it could be gynecomastia. According to one website I visited:
Gynecomastia during puberty is not uncommon and usually goes away over a period of months.

Research Antiestrogen drugs such as Tamoxifen. If you catch it early enough, Tamoxifen has proven a good treatment for pubertal gynecomastia. But don't wait! The longer you wait the worse it will get, and the likelyhood he will require surgery increases.