Hi,
Thanks for posting your query.
Your father may have herpetic lesion on tongue, difficult to diagnose and easy to treat.
High level of suspicion is required.
You should consult with physician and very effective anti-viral medicines are available.
If not herpetic lesion then there may e deficiency of
vitamins or autoimmune related disorder like
pemphigus vulgaris.
Take care,
Dr. Mayank Bhargava