Help! DS 3.7, totally dry, will not poo on the toilet.

[Butty69]

DS has been potty trained for ages, but will not under any circumstances poo on the toilet or potty, but insists on pooing in a nappy.
He totally knows and understands what it's all about. We have talked many times about it and all he says is that he's scared of doing it on the toilet and that 'it won't come out'.
He knows exactly when he needs to go, asks for a nappy, then goes away and hides to do it. Then he comes back and asks to be changed and have his pants back on.
I've tried cajolling, bribing, getting cross, but nothing works. Then he makes me feel awful by saying 'sorry for pooing in a nappy, mummy'!!
One thing I've noticed is that he crouches on all fours to do it rather than squatting, so may be that's part of the problem?
Any advice would be great, please!

Hi
I can't offer advice as such as I've got exactly the same problem with my DD age 3.2. She will wee on the toilet but will not poo anywhere other than pull-ups. We persevered with potty training trying the knickers only method but she ended up quite severely constipated as she just held it. We've taken her to the doctors and even a constipation clinic at the hospital. They advised to let her continue to wear pull ups but keep encouraging her to use the toilet which is easier said than done. They said it was a very common problem and that she will just grow out of it.

She now goes to nursery and I have to send her in pull ups because she was getting so upset at the thought of having to use the toilet there.
I have tried reward charts and explaining to her that she's getting too big for pull ups etc but to no avail. She just looks at me and says 'I know Mummmy but I love my pull-ups'. If I manage to convince her to sit on the loo she just screams when she feels it coming and gets off demanding pull ups again.

I don't know what to do next other than tell her we've run out and see what happens bujt I'm terrified of her getting constipated again as that was awful.

Hi Autmunsun,
Thanks for the reply - good to know I'm not alone! (if you know what I mean).
DS is totally aware of what needs to be done, but as with your DD, much prefers pull-ups.
I tried 'tough' talking the other day - told him that, if he wanted to poo in a pull-up, then he would have to wear them all the time. He certainly didn't want that and promptly went and sat on the toilet - but no joy!
All the information I've read says that we just need to go with it and let them decide, but, same as you, he should be going to his new pre-school (we've recently moved)and they don't do pull-ups! Daft thing is, he can't wait to go!!
Hope your DD cracks it soon - by all accounts they will do it in their own time. For now I'm giving up mentioning the toilet altogether (though he knows reverse psychology too!).