A 29-year-old man presents with 6 months of crampy lower abdominal pain, diarrhoea up to 6 times daily and occasional rectal bleeding. He has lost 5 kg. Colonoscopy shows continuous mucosal inflammation from the rectum to the splenic flexure, with biopsies confirming ulcerative colitis. What is the most appropriate first-line maintenance therapy for his moderate left-sided disease?