Talk to Gastroenterologist on Stomach Issue. Loose Motion

AOA, asking for my fiancee. he had corona in march. and he had stomach issues diarrhea n loose motion. alhamdulillah he has recovered from covid. but the stomach is constantly having issues. every week after he's having loose motions n pain. he's been taking antibiotics. which i believe more causing harm than good. but he doesn't listen. because of constant use of flagyl he started having headaches, blurry vision. now again his stomach is upset n he's gonna have antibiotics. tell me what should he do.? I've asked him to go to doc but because of his busy work schedule he's not able to spare time for himself. kindly guide.