What Causes Chronic Constipation Post An Appendectomy?

My son had an appendectomy about a month ago. He has had bowel movements since though not daily. I would estimate 3-4 days to first bowel movement and this has always been an issue with him but tonight he announced he d not had a poo for more than a week. That he is passing some diarrhoea, presumably around a constipated poo and that it hurts to push. He doesn t want to force it and is worried about hurting himself and damaging the repairing scar tissue. He also said the urge to poo comes on quickly and he needs to sit or stand still for 10 secs or more to avoid pooing himself. What is the best way to proceed? Take a high fibre bulking agent like fybogel or a laxative?