My eldest was exactly the same. We never had any accidents - ever - and he always did a poo in his pull-up the moment I put it on him for bed. It drove me mad, but I took my HV's advice (which is opposite to advice from ethanryan's HV by the way) She said that as long as he's CHOOSING where and when to go, then there's no problem at all. He has control over his bowel, he's not having accidents, and he's not going in inappropriate places. All she advised me to do was to wait, because one day, he'd choose to use the loo.
He was just over three when one day he stood up in the bath, announced he needed a poo, and I plonked him on the loo straightaway. All went well, and he never had another problem. In fact, the next day, we were in Sainsbury's and he said he needed to poo. I thought "now there'll be trouble" (because he'd never asked to go during the day, waited for the bedtime pull-up etc etc) but he went to the public loo and we NEVER LOOKED BACK!!!!!
It'll happen when he's ready, just keep asking. Once that switch in his head is turned on, he'll never turn it back off again, I promise. Good luck - try not to let it get you down xxx