There is a difference between a person's faith guiding them in their lives and that prayers are heard and answered, and outright believing God literally is speaking to you. I'm not sure which one you are saying is happening with your son.

If you believe what he is experiencing is a delusion or hallucination, perhaps you can persuade him to see a therapist or psychiatrist who can evaluate him and help him get the help he needs. If these are delusions or hallucinations (which is what you seem to be saying), you probably won't be able to convince him of that which is the nature of his condition.