His glands produced a "drain". What happens is when we hit puberty we develop glands behind our nipples. Women produce glands and breasts. Women, when pregnant produce a "stem" off their gland to drain the built up milk in their breast. High estrogen and progesterone cause this to happen on women. If he's on cycle and wasn't taking the proper ai then Gyno is a hugeee risk. Ie aromasin, adex, caber, prami, ect ect. In men, when estrogen or prolactin get to high we start getting tingling nipples or sore nipples. That is the gland behind our nipple recognizing the high estrogen or prolactin or both and developing a "stem" like a woman would, to drain fluid build up in the pec or breast of a man.
Usually once this gets to the point that the pea sized lump that starts to form causes the breast to leak or lactate, it's too late. You got full blown Gyno. You can slow it down or even control it to a degree but all in all it's safe to say your fucked and need Gyno surgery.