A 35-year-old man with Crohn's disease, currently treated with azathioprine, presents with acute severe abdominal pain, fever, and bloody diarrhoea. Examination reveals tachycardia (110 bpm), temperature 38.3°C, and diffuse abdominal tenderness. Blood tests show CRP 150 mg/L, Hb 95 g/L, WCC 14 x10⁹/L. According to NICE NG129 guidance for inflammatory bowel disease, what is the most appropriate immediate management?