My guy pee trained at around 26 months but would have nothing to do with the potty for pooping, it hurt and he wasn't having it. He began to hold his poops, and I was afraid he'd develop encopresis so went searching online for help and found it. Rewards did nothing for him, neither did praise, encouragement, making him comfortable, reading books and watching DVD's on pooping on the potty, not a thing. It just goes to that each and every child is different, what works for some won't work for all. Take a look at this site BEFORE it becomes a major problem with your son, the information and tips are invaluable:
http://www.rogerknapp.com/medical/pottytrainingrefusals.htm
What I learned from this site that worked for us was to use a "power incentive," for my guy it was a "Toy Story" DVD. A power incentive is something you retain ownership of, not your child, so it retains it's usefulness as a reward tool. The child earns 30-60 minutes of use of the item by using the potty, and not his pants or a spot on the floor or place other than the potty/toilet. I can honestly say that in just a little over 24 hours my guy was literally running to use the potty to poop by using this method!
There are also other ways given to help your child, such as allowing him to poop in a pull-up while sitting on the potty/toilet and working in steps with the goal of getting him to sit bare-bottomed and doing it, there's really a lot to help anyone with a poop refuser. You owe it to yourself to read through the info.