The most likely diagnosis in this case would be an indirect inguinal hernia.
An indirect inguinal hernia is when the hernia passes through a weak spot in the abdominal wall and protrudes into the inguinal canal. In this case, the patient has a mass that extends into the scrotum, which is located in the inguinal area, and he denies any trauma or other cause for the hernia.
The other options such as a direct inguinal hernia, obturator hernia, or femoral hernia are not usually seen in this location, and are thus not likely to be the correct diagnosis.