The testicles are located in a sac of skin called the scrotum because they are very sensitive to temperature. The temperature inside the pelvis, where you would expect to find them, is too warm for them.
It isn’t really because the scrotum is temperature-sensitive. The testicles, cradled inside, are the sensitive structures. There is sperm-making machinery inside the testicles, and they require that the temperature be kept constant. If the scrotum is too cold, little muscles around the testicles pulls the testicles closer to the body to warm them. Likewise, if the temperature is too warm, the little muscles relax and the testicles move a little further away from the body to cool down.
When the testicles need to move closer to the body to become warm, the scrotum wrinkles and holds the testicles closer to the body. The scrotum loses its wrinkles and hangs a little lower if the testicles move away from the body to lower the temperature.
If your son is about 9-10 years old, he might begin to see some changes in his scrotum. The scrotum will grow a little during these years and it might become slightly darker. Tiny bumps will start to show up in the skin on the outside of the scrotum. These are locations where pubic hair is going to begin growing during puberty.
The scrotum has an important function as the guardian of the testicles. Boys can help protect their scrotum by wearing a jock strap during sports.