A 37-year-old man with a history of ulcerative colitis (UC) presents after a recent colonoscopy. Clinically and endoscopically, he has no evidence of active disease. He has been on oral mesalamine and 10 mg of prednisone for 6 months. Attempts to wean prednisone below 10 mg result in symptoms including passing of bloody diarrhea and abdominal pain. Which of the following is the best next step in his management?