GI infection & inflammation. Or mental?

My father was experiencing lack of appetite, nausea and fatigue. His reports were more or less within normal range but in the last report, his ESR was 104. Doctor started antibiotics for 5 days and it went down to 43. He has been continuing his meds for his IBS (it's been 10 days since). However, he is still experiencing nausea and a sensitive gag reflex while having roti at dinner. He is feeling extremely down because of it since mentally he's quite depressed also after my mother's passing two years ago and not being able to go out and meet people (he's a very social person). His USG and Colonoscopy were normal other than signs of fatty liver and pancreas. He is very lethargic but I have been taking him for morning walks. He doesn't enjoy walking as he gets tired but he seems a bit charged up when guests come over. There's some stomach cramps around the upper right side of abdomen. Are his issues a physical problem or mental?