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.
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.
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.