Breast cancer in males is uncommon, especially in the young, but it can occur. A lump under the nipple of a young boy is most likely gynecomastia, which is common in boys around the age of puberty and is usually temporary.
Hello and welcome to our site and forum. I agree with Jack650's comments. This is likely gynecomastia and you can read more about it here https://www.webmd.com/men/what-is-gynecomastia#1 Please let us know your thoughts on this information.
Agree as well. In a young male, this is most likely gynecomastia, an increase in the amount of breast gland tissue, caused by an imbalance of the hormones estrogen and testosterone.