Does he smoke ?
Do those ulcer repeat every month?
What is the size of this ulcer ?
Is he diabetic.
This could be an . major apthus ulcer (Google it)
Hydrate your dad , give him lot of liquid
He needs to take rest if he is in stress.
Next Steps
visit a nearest dental college.
or send his snaps to me
Health Tips
apply dento-gel till u take him to a doctor, this might give him some relief