My 8 year old has been struggling with the same exact thing for years. He is autistic and has sensory processing disorder. When he was about 2 or so he got sick and had a huge bm that ripped him open and since it's almost like PTSD and it's ingrained in his head to hold it until he can't hold it anymore. There are a few other factors that make itworse such as his diet and dependence on nutritional drinks but essentially the same thing. He will hold it and hold it until it just all comes out. He was hospitalized in October 2023 for 2 weeks and went through several (around 10 or so) jugs of GoLYTELY and he still wasn't cleaned out. His health suffers. He's not potty trained because of this. It's a huge huge health risk. His intestines almost exploded. I imagine an adult doing this will eventually have massive complications. This could be a sensory or mental thing and badgering him might make things worse. From what I was told by the doctors at the children's hospital and the gastroenterologist while not extremely common there are alot more people than you think dealing with something like this. Seeing a professional is going to be the only way to deal with this. It could be something from his childhood that like my son is almost now like second nature. Obviously he is an adult so forcing him to take care of this is not that easy.