My son has developed a mass under his left nipple. The area around it is pinkish red. And is very tender to the touch. What could this be?

Abcess/infection. An abcess ('boil') can occur anywhere on the body and is characterized by pain/swelling/ redness/warmth. It is best to get seen and have it drained ('lanced') and possibly cultured. In some studies doing an incision & drainage (I&D) cured most staph abcesses. An oral antibiotic may also be recommended. Hope he is better soon!