Turns out (just having gotten back from visiting them) that my brother-in-law is having similar difficulties with his son. Same age, similar aspects... My nephew is basically trained on peeing, but won't poop in the toilet! He's done it three times and gotten the rewards for each time, but won't keep it up. He tells them when he has to poo and they put a diaper on him. Unbelievable!

They've given him the deadline of summer daycamp, since the camp won't take someone untrained. We'll see...