You appear to report the symptom of galactorrhea, which is milky nipple discharge when one is not nursing. The possible medical causes are many, ranging from various medications, chest wall trauma, and overexercise to hypothyroidism, liver or kidney dysfunction, or prolactinoma (a benign tumor of the pituitary gland in the brain). Your son's primary care doctor or an endocrinologist (growth and hormone specialist) can begin the diagnostic evaluation, which may include blood tests and an imaging study of the brain (e.g., MRI).
Definition - http://www.medterms.com/script/main/art.asp?articlekey=9665
Causes (see Tables 1 & 2): http://www.aafp.org/afp/20010501/1763.html
Prolactinoma reference: http://www.mayoclinic.com/health/prolactinoma/DS00532
Hyperprolactinemia reference: http://emedicine.medscape.com/article/121784-overview