Please provide some history like.....
1.are this patches on both sides of the cheek?
2.is there any bad odour or smell coming from his mouth?
3.associated with any other findings like pain,
bleeding from gums. ..etc.?
4.ask him does his cheek feel rubbery ?
With medical history,
As you described he is on medicine for
diabetes & is in controlled state.
Next Steps
get consultation with dentist for further clinical examination to clear the doubt.or visit practo for the same. We are here to help you!
Health Tips
don't worry about it. These are not black nerves .
but for further information about it , please get consultation with the dentist.
Thanks!