DS born on March 2009 & having a hard time with potty trainning

[mamiof4]

Hi,

My ds is 2yrs and he just started showing intreast in the potty & going pee pee but still refuses to go poop in the potty. I will have him in underwares all day & will stay dry in them but wont poop til I put on a pull up to go to sleep then thats when he will poop.

Anybody have that problem?

Any ideas?

Rosie