Oh the relief at knowing others have this too. Mine is 4.1 and has been learning to use the toilet for about 2.5 years now! He was almost done by 2.1 (early starter but nursery assured us he was ready) then we moved house, then his baby sister arrived, then he moved rooms in nursery, then his key worker left, now he's started big school. At every new event/change he had a regression, and sometimes in between.
Most recently, at preschool, he'd have to five wee accidents some days, none on others, most accidents days were when he was with younger children in a classroom where he used to be in nappies IYSWIM. Since starting school he hasn't had a single wee accident (yet - I'm a realist). This is a huge relief.
HOWEVER, he's come home almost every day with poo welded to his bottom. Just skid marks often but sometimes a bit of a covering. He absolutely will not tell anyone that it has happened so its only if they're changing for PE that they notice. Or he tells me when I collect him.
He does this at home also, I know exactly when he's doing it and he will flatly deny it to my face even though we both know what is happening. Then screams because his bottom is sore he doesn't want me to clean it. Liking the idea of cold shower for sore bottoms plus making it faffy for him so he gets bored - if it is indeed behavioural.
I echo other posters - its SO frustrating. And SO difficult to contain that frustration (trying not to give him psychological issues). DH in particular struggles with this. My only consolation is that it must end at some point and that several months ago it would have been whole poos.
I'm wondering if it might be worth a trip to the GP to check for constipation again, having read the above and other threads which talk about distension of the colon from long term constipation meaning they can't tell when they need to go etc.
Otherwise, perhaps a regular dose of prune juice!
Seriously though, thank you for making me feel better. MN rocks!