A 35-year-old man with Crohn's disease, currently treated with azathioprine, presents with a 3-day history of worsening abdominal pain, bloody diarrhoea (6 times/day), and fever (38.5°C). Stool cultures are pending. Examination reveals diffuse abdominal tenderness. His CRP is 150 mg/L. According to NICE guidelines, what is the most appropriate initial management for this acute flare?