My Son 22 Years Old. He Has Been Down And

My son 22 years old. He has been down and out for the last 8 months. He had mono and was tested again and was negative. He has had many tests and now he just got results back for his stool culture. They found two bacteria in his stool culture which his doctor has him on Bactrim . He freaks.out anytime he has a bowel movement that is a different color other then brown???? He ends up very argumentative and very mean and loud to his mother. Any suggestions or ideas. This has been going non to long.