S.
My son was afraid of sitting on the potty rings. He felt very unstable and thought that he was going to be "Flushed Away" like the movie. He did not go poop on the potty until I bought the potty pal from target.com. It is a toilet seat cover with seat and potty seat built in. In other words, it looks just like the one you have now but also has another pull down with a kids seat. It does not move so it eliminates the unsteady feeling. It only cost around $30 and worth every penny. We bought two, one for upstairs and one for down. Our boys are now nearly 5 & 6 and we still have them on our toilets.
As far as rewards go, we hung candy in the bathroom, two different kinds. They would get one for going pee and two for pooping. We also put a reward sticker chart on the bathroom mirror. I just made something up on the computer using their names and each success would earn them a sticker on the sheet. Pooping is always the hardest part, stick with it and it will come - I promise!! Good luck!
Ooh and don't do the pull up thing. Use regular underwear during the day and if he poops just dump it in the toilet with him and tell him that is where poop goes. I used pull ups with my oldest and it took longer than going straight to underwear and dealing with accidents.
Wishing you quick success!!