My Husband Has A Colostomy And Has Had Dirreahea For

My husband has a colostomy and has had dirreahea for 3 weeks. Went to the doctor and he checked for something that's caused when a person takes a lot of antibiotics, which he has taken. but the test he did turned out negetive, But he didn't suggest anything to help him. i have been giving him Imodium and pedialite, and feeding him rice, but nothing seem to stop it. What else can I do?