Given the history mentioned above, a lot of anxiety, rigid belief and possibility of influence of depression and mild dementia changes in his brain if he doesn’t have any history of psychiatric illness in past.
Next Steps
He needs to be consulted with a psychiatrist. SSRI, low dose antipsychotic and stimulant can do wonders. Also assessment other possible differentials should be kept in mind if no past history of psychiatric illness. With the given history likely illness anxiety/ depression with somatosensory preoccupation.
Health Tips
Don’t try to prove him wrong, that may only worsen his mood and behaviour may get agitated. At his level of perception and thought, he would feel that something is wrong with his health like a real fact. Thus countering him won’t be of any help.