It sounds like he needs more fat and less fiber. Using fiber to stimulate bowel movements is like using a plunger to unplug your plumbing. It works but it stresses the plumbing. He can test this theory very easily by taking an oil-based laxative. If it makes it easier without pain, he needs more oils and fats in his diet. Both my parents had medical issues from impacted colons in their later years, so I learned about it. There's a book called "the fiber menace" which will scare people about too much fiber, and while I thought he was rather extreme about it, the author made an excellent argument about how fat is better than fiber for your digestive system.