It's very reminiscent of a shunt line. The treatment for hydrocephalus when he was young would have been to place a catheter in the ventricle of his brain and run a line just under the skin either to his heart or, more commonly, to his peritoneal space.
Sometimes a small pump, looking like a button under the skin, was used to connect the catheter to the line. It could be gently depressed to prevent clogging of the line.
The purpose of the system is to reduce the pressure caused by excessive accumulation of cerebrospinal fluid, the liquid inside and surrounding the brain, preventing pressure damage to the brain.
Hydrocephalus occurs commonly at birth and in some congenital disorders. But it also can occur later in life from trauma or a subarachnoid hemorrhage. In those situations, the trauma or SAH will impede the usual flow of fluid and result in increased pressure.
When this issue came up before, I posted something similar to this but no one responded. But that's what it looks like to me.
Have we ever seen any of his medical records?
His head did look larger in some younger pictures.