Some kids just have a hard time with it. The reasons vary - some don't like the "splash", some don't want to let the poop go, some are just afraid, some haven't figured out how to poop sitting down.
I'd say be patient.
Keep talking about it, asking him where mommy and daddy poop.
Make playdoh poop and have fun putting it in all sorts of silly places. If you have a little potty, make sure you include that. I don't recommend putting playdoh in the real potty though - it might clog the plumbing and could teach him that it's OK to put toys in the toilet.
If he has regular times he goes, see if you can get him to sit on the potty at those times. Even if he doesn't go, reward his efforts.
See if he'll go in a diaper while sitting on the potty. Then you can try with the diaper unfastened, in the potty (if it's a little potty), then no diaper at all.
at 3:34 PM on Feb. 25, 2009