This question has come up numerous times. You should look up old answers because you will find even more child-centered approaches. I don't believe in the "taking control" approach. There are three things that a toddler has absolute control over--eating, sleeping and pooping. Everything else is in the control of his parents. We have to allow them to make their own choices about their body functions. We can present them with healthy choices to eat, but we can't force them to eat. We can tell them it is bedtime, but we can't force them to go to sleep. We also have to allow them to use the toilet when they are physically AND emotionally ready. Otherwise it could backfire and cause problems in the future--embarrassment over pooping, holding it in, etc.
If it is any consolation, my almost 4-year old daughter still poops in a diaper, despite peeing in the potty since she was 3.5. She has a personality (as well as being age three) which has a need for control. I think this is spilling over into her sitting on the potty to poop. She just can't let go of her muscles enough. I'm trying to be patient, but it IS difficult and I'm tired of changing diapers. But I trust that she will someday do it.
Good luck, Laura