Hi R.! This is a tough one, as children this age can be real control freaks (I mean that in the most loving possible way :) ), and they will latch on to things others can't force them to do - poop, eat, sleep, you name it - to exert their control over their own bodies.
In general I would recommend loosening up and allowing him to poop in a diaper if that's the only way he'll go - without judgment. A lot of us rush to potty train our kids because of peer pressure or the pressure the preschools put on us. It sounds like your son is fully capable of going on the potty, but he just doesn't want to "let go". This is not something you can push or rush.
There's a great potty training technique out there that involves having your child teach a stuffed animal or doll how to use the potty, and after that they will generally do it themselves. It involves candy bribes too, which I know people like to avoid but in extreme cases can be really helpful! There are several books out there on the subject. Try this one:
http://www.amazon.com/Toilet-Training-Less-Than-Day/dp/06...
Some of the "train in a day" books teach you to have "potty parties" - avoid those for your son, they work for some kids but my instinct tells me that if your son is working on control issues, a potty party will just scare him off.
I hope this helps. Good luck, and hang in there. This is a major milestone for kids, and some just need to do it at their own pace. He'll get there!