I'd say that is possibly what's wrong then.hes done approx three in the time ive been potty training, since just before he was 3, they were quite big and he did say they hurt his bottom, this is where the issues start, after that pain incident, i give him weetabix for breakfast most days and loves fruit, and a fig roll every couple of days as i was worried that he wasnt eating enough fibre and that keeps him regular, but he holds it in. I have taken him to sit, read a book, took the ipad, but id say 90% of those times hes got so upset i didnt want to push him too much incase he develops a major fear the other 10% he sits there and doesnt do anything.
That one time sticks in their head and they associate it with it every single time.
I have no advice on how to get past it as I think they just kept going. They did contact the doctor and they put him on some kind of kids lactulose so that it didn't happen again and eventually he learned it wouldn't be painful