You have mentioned about undescended testis since birth. This is more common in premature babies. This can affect one side or both sides. The undescended testis remains anywhere in its normal path of descend-from abdominal cavity to inguinal canal. Naturally by one year of age the undescended testis reaches its ultimate destination. If not, surgery is needed to pull the testis to its normal position and fix it. Operation is normally performed between 12 to 15 months. As your son is now 17 months old, it is the high time for doing surgery. Another options like hormonal therapy is available but effectiveness is not proven yet. So surgery is the treatment of choice for your boy. You better consult paediatric surgeon or urologist.