A 66-year-old man with known metastatic prostate cancer receiving palliative care develops severe constipation despite regular lactulose and senna. He is taking regular Morphine Sulphate Modified Release (MST) for cancer pain. On examination, the abdomen is soft but there is palpable stool in the left iliac fossa. Rectal examination reveals hard stools. What is the most appropriate next step in managing his constipation?