If stomach pain of your father increases on eating food, it suggests Duodenal Ulcer
as one of the commonest cause.Whenever food passes over the ulcer, it causes stimulation resulting in pain.
Other causes are:-
It is suggested that have an Endoscopy
done. This will help in confirming the diagnosis.
If Duodenal Ulcer is the cause, get medication for H.Pylori, if any and antacids.