No details about nature of cancer , medications given, past and present investigations more particularly reg stomach ulcers. A proper Upper GI endoscopy if advised by a Gastroenterologist and his opinion on management will be helpful . Confirm none of the medicines given a cause. He should be given bland diet more frequently so that stomach is not empty or over filled for long time. Pantprazole 20 mg twice a day for about 4-6 week should help along with Antacid liquid containing local anesthetic like Mucain Gel . Enzyme medications like Digeplex liquid or Unienzyme etc. should help. If no relief consult your regular doctor or a Gastroenterologist