I am at my wits end. My son is going to be four years old next month. He has been potty trained (pee only) for almost a year but he absolutely refuses to poop in the potty. He has only pooped in the potty once ever and it definitely wasn't on purpose. I am looking for any advice, because of the obvious - cleaning up a big kid's poop is even grosser than dealing with baby poop.

Here are things we have tried:
- rewards. I tried giving him a small reward every time he sat on the potty. It made no difference. I tried buying him a toy he really wanted and promising he could have it if he pooped on the potty. He still hasn't earned it.
-routine. I've tried just having him sit on the potty several times a day. It makes no difference. He will poop in his pants five minutes after sitting on the potty
-talking. I have talked to him about the potty, about how all big boys poop on the potty, about why he doesn't want to. He says he is scared of the potty and that he doesn't want to poop on it and never will.

Physically he is fine. He is not constipated. Pooping is not difficult for him. He goes to preschool 3 mornings a week with no problem - he just never poops while he is at school.

I honestly have no idea what to do. It is absolutely impossible to make him poop so it is a battle of the wills and he is winning. Everyone says, oh don't worry about it. They will be potty-trained by kindergarten. I am seriously starting to wonder.

Edited to add:

We have also tried books about potty. Watching mom use the potty. And even making up silly stories about how the potty wants to go in the potty with its friends.