The chances of fertility with a single normal testis are similar to those males having two testes. The main concern should be whether or not the testis that has been brought down is normal. Many of the undescended testes are not completely normal. Protocol biopsy from the testis, done at the time of bringing the testis down, usually is able to tell whether the testis in question has a normal architecture. Functional normalcy of this testes, unfortunately, cannot be known until puberty. Regards.