I totally understand what you are going through as I have issues with my (almost) 10yo. One of the ways I was able to get through to him was talking about how it hurts but it hurts more in the tummy if he holds on, and that once he's done it he will feel much better when it's out. But not when he's upset because they are not in a good place to discuss stuff then We had problems with DS1 until he was about 5 and still do if he has a gastro bug that upsets his system.