Hi and welcome to Healthcaremagic. I understand your concerns and I will try to help you as much as I can. This is probably caused by Trimetoprim antibiotic which is known to cause such difficulties and abdominal discomfor including diarrheas. SO you need to take probiotics or switch antibiotics. Other causes include dietary or infectious ones. If this is caused by diet then he should try to rest more and switch to a diet of clear liquids. Drink water, juice, bouillon and weak tea to avoid becoming dehydrated. Replace lost fluids and electrolytes with sports drinks. Avoid coffee or soft drinks that contain caffeine, since caffeine increases the loss of water and salt. If you have nausea in addition to diarrhea, take very small sips of fluid frequently and suck on ice chips. You should also consider over-the-counter medications such as bismuth subsalicylate (Pepto-Bismol) or loperamide (Imodium).In cases of infectious diarrheas, you should do microbiologist testing of stool and see the exact microbe and then antibiotics can be used additionally. I hope I have answered you query. If you have any further questions you can contact us in every time.