He is 11 years old. my son has got a small hard lump under his arm. what is it?

Lymph node: A hard lump under the arm is most likely an enlarged lymph node, there so many causes for enlarged lymph nodes most common is infections, local or systemic, but if it is only one gland it is usually due to local infection, you need to take him to his pediatrician to check him, make proper diagnosis & R/O other causes for this lump. Good luck.

See his MD/nurse: There are a lot of reasons for a child to have a lump under the arm. It usually comes from the body fighting germs from a skin scrape or infection. Things that are important are how long it's been there, does it hurt, is it red, is it getting bigger, and are there other lumps hiding elsewhere (along the neck, groin, other armpit, etc). It's important to talk to his doctor or nurse about it.