I don't really have any good advice, other than to just be patient. He'll get it. My son did the same thing. He potty-trained just great, just before he turned 3, but would ask for a diaper to poop in (for about 4 months). I tried lots of different things. Finally one day, he had diarrhea and I gave him a diaper, but it ran down his leg and I told him that diapers don't work for diarrhea, he has to use the toilet. He did, and that was that! After that he never asked for another diaper. I never really pushed it, and we never had any issues, nor any accidents. All kids are different, but for us he had some 'fear' about it and I know there was also a power struggle going on. I know it's so frustrating though! I did read the article provided below and you could certainly try some of the things suggested. If he resists though, I wouldn't push it. You could create even larger problems. Best of luck!