hello, There are two reasons for his shortness of breath, one is afib and high heart rate, other contributing factor is stomach problem. Stomach problem is mostly it s a gastritis and acidity problem. Does he have upper abdominal pain, nausea, bloating, burping, increase in pain on food, sour water feeling in throat or chest burning, if yes it further supports the diagnosis. He should avoid fatty, oily and high calorie diet. Have some walk after having food instead of taking rest. Have multiple small meals instead of heavy meals. Have regular sleep habits and avoid stress. Lots of green leafy vegetables, fruits. Avoid smoking and alcohol if any. You can get prescribed tab Pan DSR 40 mg beforebreakfast once a day for 2 weeks. Also, he should consult the doctor in order to increase the dose of rate control medicine, so that it will relieve his shortness of breath.