Potty strike

It's actually rather endearing (in a way that only parents can use the word "endearing"!) While pooping, Rowan has a spot behind the couch where he goes for privacy while doing the doo in his pull-ups. But he refuses to try it on the potty. What gives? The pee-pee situation is ok, he even stays dry all night some days. But why the block to going poo-poo in the potty?
Any ideas on how to get him to the next level?