My DS was the same with poos, we would find him hiding somewhere do his poo in his undies. I found he was scared of doing it on the toilet, so for a while i would sit and hold his hands or talk to him, when we had success we all cheered and he got to phone Nana with his news......lol poor Mum.
My DD started a sticker chart for him and he would get to pick a sticker to put on his chart every time he did a poo, this worked for him, i must say though we still have the occasional accident.