Hello MotherofSon, Welcome to HealthcareMagic Forum.
is a fluid-filled sac within the scrotum
. Just take him to a General Surgeon
and get him examined by him/her to rule out if there is Inguinal Hernia
. They are usually not dangerous and are usually treated when they cause discomfort. In your Son's case, may be the sac was not closed Properly, Sclerosing that is thickening or hardening medications may be injected after aspiration to close off the opening. This helps prevent the future build up of fluid. Kindly take him for the follow up and Discuss the Treatment Plan with your Doctor.
I wish him Good Health.