my alsmost 3 year old refuse to poop in a potty and does it in his pull-up only during his nap

My son has been successfully potty trained in regards to peeing. He tells us every time he has an urge and doesn't need pull ups during daytime. Quite the opposite is true about his pooping. He never tells us, goes to hide in a corner and does it in his pants. We always tell him that this is not OK and he needs to go on a potty like he does when he wants to pee.
I would put him on a potty just before his nap time, he would sit on it for 30 minutes, and he wouldn't poop. Then 5 minutes later as soon as I put him in his pull-ups to put him down for his nap, he does it immediately! A few times happened when he smeared his poop all over his bed, toys, walls.....an absolute disaster! I get really upset and have hard time to stay calm because it seems he does it on purpose. We have been through explaining him what he needs to do a thousand times. I can tell he feels bad but then next day he does the same thing!
Please advise me what to do. I am at the end of my wits.

[teddybears4ever2]

Well you can always think about giving him a reward for pooping on the potty.  You could buy him a toy he wants, take him out to eat, give him candy, or whatever else he likes. That might help.