What Causes Diarrhea After Removing Stent To Drain Kidney?

My son had a kidney stone in September 2013 and a bad kidney infection. They needed to insert a stint to drain the kidney. He was on IV for a strong antibiotic for 4 days and after on a tablet (I don't know the name). After the stint had been removed 4 weeks after the operation, he started to have diarrhea and still has it (5 months). Could you please let me know what should I do?