How about getting him a special toy that he is only allowed to play with after going poop in the potty? It has to be something wothwhile (m&m's were not a special enough reward for my son) and immediate (he won't really associate a future trip to Wild Adventures with going poop in the potty). Here's what I did:
My son was competely trained on pee-pee but he would NEVER go poop in the potty. He only wore pull-ups at night, so he would either wait until he had the pull-up on and poop at night or he would hold it as long as he could, and then he would ask me to put a pull-up on.
We bought him a special toy (nothing outrageous, but special and different than other toys he had - a toy train for about $25) and we showed it to him in the evening (around the time he would normally ask for a pull-up). He got very excited, and then I explained that we would only open the toy if he went poo-poo in the potty. He decided he would try, but he didn't go (I think he thought just trying would get him the toy) so no toy. He threw a fit of course, but we put the toy away unopened. He was determined though, and tried again, and sat on that potty until he went. We sang the potty song (something I just made up), did the potty dance, gave him the toy and made a really big deal out of it.
At bed time, we put the toy away, and told him that it was special and he could only play with it after going poo-poo in the potty. So, the next night he was excited to go poop in the potty and get to play with his toy again. He liked the idea that it was special that way. Also, I made sure that his little brother was not allowed to play with it, so that made it even more special to him. He went on for about a week or so and eventually stopped asking for the toy after every poop, but continued to be fully 100% potty trained since then. I think he just had to get used to going poop in the potty.