Hello,
if it's not during an act or a performance show (as per entertainment).
I would highly suggest to refer him to a psychiatrist and have him take some psychological test and some blood test.
Quote : "Hearing voices is an auditory hallucination that may or may not be associated with a mental health problem. It is the most common type of hallucination in people with psychotic disorders such as schizophrenia. However, a large number of otherwise healthy individuals have also reported hearing voices."
Here is link to give you some guidance.
https://www.mentalhealth.org.uk/a-to-z/h/hearing-voices Best of luck.
Cheers