A 28-year-old man from Delhi presents with a 6-month history of bloody diarrhoea, abdominal pain, and weight loss. Colonoscopy reveals continuous mucosal inflammation limited to the colon with crypt abscesses and surface ulceration. Biopsy shows inflammation confined to the mucosa and submucosa without transmural involvement. Laboratory tests show elevated CRP (8.2 mg/dL) and faecal calprotectin 320 µg/g. He is currently on oral mesalamine 2.4 g/day with partial response. What is the most appropriate next step in management?