When my son turned 3, everyone was on my back that I should put him in some sort of pre-nursery program so he could start to learn to be away from Mommy a bit. So I found a little program at a nearby church, 3 hrs 3 days a week. When I told the director of the program that he wasn't potty trained yet, she looked at me like I had 20 heads and said he HAD to be potty trained before he could start the program. Soooooo, I chose a week shortly before the program began to introduce him to underpants, the potty, going number one, number two, etc. Long story short, he took to peeing in the potty like a champ, but pooping? It turned into a 6-month long power struggle, with me growing more and more desperate because he was slated to begin regular nursery school in the fall and this was FULL day, 3 days a week, also potty trained required.
Like you, I tried the sticker charts, M&Ms, little prizes, all the tricks that seemed to work for everyone else to no avail. He simply wasn't interested.
My absolute low was the day he pooped a pair of new underwear and smeared it all over his room. I made him help me clean that up, believe me.
Anyway, here's how I solved the situation. I took him to the toy aisle at KMart and said you may pick ANY toy on the shelf, regardless of price, on the condition that you cannot have the toy until you poop on the potty. I put the reward under his control, rather than choosing it out for him. He picked a Transformer set with two characters, probably around $30 or so. When we got home, I made a big show of putting the toy way up on my closet shelf and told him as soon as he pooped on the potty, he could have one of the Transformers. No joke or exaggeration, I laid down for a nap about an hour or so later, heard him get up from his nap, tiptoe into the bathroom, sit on the potty and bingo! He pooped. Totally knew how to do it the whole time. He was just too stubborn until I let him choose out for himself the one thing he wanted most.
Since he had chosen a set of 2 Transformers, we played out the exact same scenario the next day with the second toy. On the third day, he - being no dummy - asked what he'd get if he pooped on the potty again. I told him if he did it two days in a row, I'd take him to McDonalds for a Happy Meal. After that, I continued to space out rewards for about another week, each smaller than the one before, and then no more rewards after that. Just big boy underpants and praise. He's never had an accident since (he's 8 now and would be completely embarrassed to know I'm telling this story).
This exact scenario may not work for you and maybe your son is still not entirely ready yet, but look for that thing - toy, trip, experience, restaurant, whatever - that he wants so much more than playing the power game. I'm usually the last person to advise bribery of any sort with a child, but this was one instance where it was soooooo worth it.