Based on what you describe, your son may have an undescended testicle, one that has not moved down into the scrotum where it belongs. This is the most common disorder of male sexual differentiation, occurring in about 30 percent of premature births and about three to four percent of full-term baby boys. Usually the condition corrects itself during the first six months of life, and fewer than one percent of all cases remain a problem after six months of age. In another condition, retractile testicle, the testicle resides in the scrotum but may be drawn up toward the abdomen in response of touch or temperature change - the result of a hyperactive reflex. This problem usually corrects itself before adolescence.
If your son does have an undescended testicle, it is important that it be treated since this condition presents a risk of infertility, testicular cancer, hernias and testicular twisting (torsion) later in life. A partially empty scrotum could also cause your son psychological stress as he matures. You should know that the chance of spontaneous descent decreases markedly after the age of one year.
Your best bet is to take him to a specialist (a pediatric urologist) for evaluation and treatment. In most cases, undescended testicles can be corrected by a surgical procedure called orchiopexy. This operation will lower your son's risk of infertility. (Normal sperm production requires the cooler temperatures of the scrotum.) It is done under general anesthesia; usually children can go home the same day and will be back to normal within a day or two. While the surgery doesn't reduce the risk of testicular cancer, it improves the chances of early detection. The only other treatment option, injections of human chorionic gonadotropin (hCG), succeeds in only 10 to 15 percent of all cases.
Andrew Weil, M.D.