I had this problem with my son. He wanted a diaper, would go in the corner, did not want to poop on the potty. This also turned in to him holding and holding his poop, he would go sometimes over a week without pooping. It was awful, laxatives, etc so if you can nip this in the bud, do it! What I finally found that worked was whatever motivates your kid. Each kid is different. I am just now done potty training my daughter and we did the princess thing, with my son it was spiderman, but use whatever you know motivates your kid. I made a big poopy chart (for both kids during their potty training, as it happened with my daughter too, but I nipped it in the bud before it became a holding issues) We made a big chart and I got whatever their favorite stickers were, posted it in the kitchen for all to see. Made out a number of boxes up and down (this is totally up to you) When they pooped x # of times, they got a prize. Sticker for each poop, we used 5, then they got to go to target and pick out a toy (within reason of course) The first prize was a Spiderman toy for my son (we actually bought it and I put it in the bathroom up on the towel rack where he could not reach it, could not touch it, just admire from a distance until he pooped on the potty) with my daughter, it came together much quicker, she picked out a sleeping beauty doll, then next prize was a piece of candy, it gets smaller just by itself. Still I use the chart when things come up, like we were at a neighbors last night and I knew she needed to poop, was acting "weird" so I told her she would get 2 stickers when we get home on her princess chart for pooping "out". It worked like a charm. My son still uses his "big boy" sticker chart (he is now 5 1/2 and in Kindergarten) when issues come up from time to time, we revert back to the chart with stickers. It really helped me out and luckily my daughter (almost 3) totally bought into the concept as well. You can also motivate with underwear that they love (find the kind that really does it for them) for my daughter it was "you don't want to poop on sleeping beauty, that's yucky" and it worked. My son would just hold until we had all sorts of problems, so avoid that at all costs, although it is VERY common for boys to "hold" I hope this helps, sorry it is so long. Best of luck and happy to help if I can.
C. in Alpharetta