Thanks for your replies. Not a medical problem. A stubborn, control problem. It helps to know that others have had similar situations. I'll share this with his parents of course. We'll keep on working at it. This too, shall pass. (pun intended)
Just keep an eye on it. Holding can develop into a stretched bowel and ignoring signals you need to go to the toilet can result in those signals reducing to the point you get about 30 seconds warning - not good when you are out and about. After my experiences with ds9 I would err on the side of caution and check for medical or emotional reasons first.
