Hi Suzanne,
You might be surprised, and honestly, I'm surprised too, at how common this issue is. I won't call it a problem because in reading these potty training posts, it seems all too common--I'd then call it a "phase."
My youngest daughter was more than 3 years old before she began pooping in the potty. I used the sticker rewards system(and a silly song about pee-pee in the potty) to get her to pee in the potty, and she did it amazingly fast, but pooping was one thing she WOULD NOT do! I think she was afraid to go poop in the toilet, or maybe like Marda said, it was a control issue for her, I'm not sure.
But I will never forget the day it just "clicked" for her. We were in the pool at the hotel when we went to go visit daddy in Yakima while he was working. She had on a Little Swimmer thing under her swim suit, and we were just goofing off in the pool. All of a sudden she says "Mama, I have to go poop." Real quiet like. It almost didn't register what she said, and then I was like "you have to what? You DO?! Okay, let's go quick!!" And I hustled her out of that pool and back to the room as fast as we could go.
I was in such a hurry to get her onto the toilet so she could poop that I ripped her suit, but I made such a huge and happy deal about it and her accomplishment, that neither of us minded the hole in the suit lol. From that day on, she was officially potty-trained!
My point is that I think rewards work to a certain extent, maybe for some kids they work to help them do both more than they work for others, but I don't think I would show him the bike. I think that at the phase he's in, I would let him go in his own time. For better than a year, I had asked my daughter if she wanted to go poop on the potty, had done the whole waiting forever while she sat there, and nothing happened, and finally, I just quit pressuring her, only asked occasionally, and when she finally did, I praised her immensely. The smile on her face that lit up the world when she finally did it and realized she *could,* well, THAT was so worth the wait.
Like one of the other Mamas said, he won't be going to kindergarden in a diaper. It may be frustrating and seem embarassing to still have to buy diapers when your child can feed themselves, but it's more common than you think. Let him do this on his own time, and don't pressure him so much. It WILL happen. Give him the bike after he's done it, and not a a reward. Just give it to him to have, as a nice surprise.
Good luck,
K. W