Wow, B.... it sounds like a very complicated situation for both yourself and your son. Please take a step back and reconsider.
It sounds like your son is not motivated to use the toilet for poop right now, and that you are sort of driving the train on this. Let me point out that he's not terrifically behind the rest of the group by any means--in fact, there are many children who struggle with using the toilet for poop at this age. Accidents are more than common.
That said, you should also know he could be experiencing some pretty frustrating and angry feelings about this, which will continue to develop into a larger power struggle. Children do not respond well to having others attempt to control their biology, and by choosing to reward him for "producing" in the right place while showing him something he can't have is sort of akin to being an older sibling and dangling a desired toy in front of him, then whisking him away. His sense of anger at this situation may be fueling this struggle. He may also have some other, unaddressed issues with using the toilet for poop and be frustrated that he 'can't' earn the toys.
If it were me, I'd take a huge step back. Give him the Hot Wheels because you want to (which is the only reason we should offer kids toys, not because they've earned them--way too much stress for kids!) and instead, tell him that he's going to be in charge of using the toilet. Don't remind him, don't engage in the back-and-forth of "you need to go"/"no I don't". Please believe me, I've seen this countless times: the more you can stay emotionally detatched from the pooping, so much the better. I'd even offer diapers or underwear each time, without a lot of fuss, and see what he chooses. When he has accidents, take him into the bathroom and help him clean up, but make sure he takes an active role in the cleanup. (Not touching the poop, of course, but holding open the plastic bag for dirty clothes to go down into the laundry, holding doors for you, coming down to the laundry room...whatever you do.)
Some parents buy cheap underwear and just trash them, others choose to allocate a certain number of underwear a day (say, three) and once they are used up, "Well, we have no more underwear today, so we'll need to use diapers." This isn't meant to be a shaming technique, but is more for the adult to keep their sanity, so set your limit for what you feel you want to do.
For what it's worth, I HATED changing poopy underpants, but noticed that the more I got upset and made it my problem, the more inflamed the situation grew. Who knows why kids sometimes get into this habit? It was only when I became nonchalant about the whole thing and let the child in question have space to become upset about it did things start to improve. When we let them work through this big stage of development in their own time, things go much more smoothly. Stay away from the punishments and rewards and give it plenty of time (I've seen kids need months to make this transition).
More info: assessments can be helpful. I love this site:
http://betterkidcare.psu.edu/AngelUnits/OneHour/ToiletLea...
Good luck!
H. Wheeler