Talking to self may be a symptom of psychiatric problem, however some more details will be needed. It can be a symptom of neurological disorder too. He will need a detailed evaluation, and family members who live with him like wife and son will be needed for detailed history
Next Steps
Consultation with a psychiatrist and a neurologist both will be best
Health Tips
1.
Vitamin B12 levels and
Thyroid profile should be done, other tests may be advised after consultation.