Your prize system is apparently not balanced well enough with your son's enthusiasm, and thus you've set up unrealistic expectations (the prizes are too valuable to your son). One of the biggest problems with prizes or bribes is that the reward for a job well done remains external. He can see that you are excited and proud for your son when he poops. It might help to ask him how excited and proud he is when he succeeds. Over time, that should make a difference.
Explain that you are worried about him hurting himself, and that you need to revise the prize structure. Give smaller prizes or sporadic prizes (google "intermittent reward" for the scientific basis for this), or at least a maximum of one or two prizes per day, depending on what his "normal" poop schedule would be.
He may be unhappy about this and regress in his training, but if that's the case, then the whole basis for the training is psychologically off-kilter. You might consider giving it a rest for a couple of months, and then reintroducing the idea of using the potty again. Don't worry too much about derailing the success you've had so far. This is like laying a good foundation, and you'll all do better in the long run if it is straight and sound.
And for most kids, success with pee comes first, and success with poops is a separate stage. It could be that your son can handle both, but keep in mind that it might be too much to handle all at once.