How can we encourage 3.3 year old nephew to poo on toilet ?

[Tillyboo]

My nephew is very bright for his age but his mum and dad have had problems persuading him to have a poo on the toilet. He always wants to poo in a nappy although he'll wee quite happily sat on the loo.

Has anyone got any tips to help my sil & brother get him through this phase and enable him to feel happy pooing on the loo ?

Thanks

Same with ds1, one day though he just climbed up, sat backwards on loo and did it. Has never looked back!
I think the more of an issue it is made into, the less inclined they are to do it.

I think it must be a common thing that they just don't feel safe/comfy on a big toilet for pooing.

...btw he still does it facing the cistern!