I would advise you to visit the ER at the earliest so that your father can be examined and undergo investigations such as urine routine, serum electrolytes, etc. and find the underlying cause of the problem and get treated accordingly. He may not be able to follow his regular diet of 3 square meals as he may have developed gastritis, kindly breakdown the meals to 5-6 small meals in a day, kindly give him a liquid diet or a soft diet that he may be able to tolerate as he may have indigestion. If the problem persists or worsens visit the ER at the earliest.