Does anyone else have experience of this please? It's starting to really upset me 
My 3.3yo ds is deliberately holding his poo in when he needs to go. I know because he suddenly stops what he's doing, screws his face up, clenches his buttocks and walks on tip toes. He then sucks his thumb and asks for a cuddle and seems upset.
I've tried talking to him gently and explaining the importance of going when he needs to, but it's not working. He's only tending to poo once or twice a week,and that's after I've spiked his yogurt with Lactulose.
He's always had good control in that dept, and has seemingly always only ever pooed at home. I made sure we didnt go anywhere for a few days last week for this reason.
His belly is now sticking out/is hard, and it's affecting his sleep.
What should I do?