Q&A: When should we treat our son's undescended testicle?

I agree with the advice you've been given and this is why:

At birth, three-to-four percent of full-term males have one (or, rarely, both) testis that have failed to complete the normal migration from the abdomen into the scrotum. In premature babies, this number is even higher. By about three months, most of these will complete that movement into the scrotum, so by one year of age, only about one percent of boys have a still undescended testis. At that point, it is very unlikely it will descend on it's own. Testis that remain too high up within the body have been shown to produce fewer sperm, and, more concerning, have a higher chance of developing a tumor. For these reasons, surgery is recommended in the second year of life to pull the testis into it's proper position and secure it there.