May be he had unknowingly bitten his tongue- that has resulted in a small blood collection and pain- due to which he refuses to eat.
Or he might be developing an ulcer on his tongue. If associated with fever
it might be due to a virus.
Just give him bland food, fruit juices, soups, milk, curd etc. You may also apply mouth gels like zytee to reduce the pain. He will be okay soon.
How-ever if its really bothering him, he refuses to drink even, develops high grade fever or skin rash
, please do not hesitate to see your Doctor.
Hope I have answered your query.
Wish your son good health.