I assume it cannot be surgically-improved or he would have tried it by now.
“Robert F. Kennedy Jr. has a medical condition called spasmodic dysphonia, which affects the muscles in the voice box and causes the voice to sound strained or hoarse. It causes involuntary movements or spasms of the vocal cords.
The exact cause of SD is unknown, but it is thought to be related to abnormalities in the basal ganglia, a region of the brain that helps control movement. SD is a rare condition, affecting fewer than 1% of people with voice disorders.”
From the Google search