Hi thanks for your question.
Stomach pain and vomitting after operation could be due to gastritis(inflammation of stomach),secondary to NSAID(pain killer) taken for post operarive pain.
Stop pain killer if he is on still taling..
Ask him to avoid spicy food.
Your father need antacid
and proton pump
blocking(acid blocking) drugs like pantoprazole
Consultation with a physician is encourged.
Hope this answers your question.