It's very common for young boys who are going through puberty to have some swelling or enlargement of the breast tissue under the nipple. This is the case for about 70 percent of adolescent boys. It usually occurs on both sides, although it can occur on just one breast.
The cause is the hormonal changes that occur during adolescence. There is a slight imbalance between testosterone and estrogen production for a short period of time. It's not clear why it sometimes affects one breast more that the other.
Usually, this breast enlargement regresses (goes back down) as the other pubertal changes progress. There are no special treatments needed. If there is any discharge from the nipple, or if your son is not showing other signs of going through puberty, then further evaluation could be needed.
There are reports of breast cancer in men, but it is extremely, extremely rare in adolescent boys, and I wouldn't worry about it.
You can read more expert advice about breast development in boys here.
