M.C.
My daughter did this for a solid YEAR. Just encourage him to do the diaper thing, because he is having some kind of mental block about pooping. You can continue talking about it, reading the books, telling him when you are going, etc. but don't push him to poop in the potty, because my girl held it for days and became impacted. we had to use suppositories and give her fiber laxitives for a long time. She is 8 now, and she still will get constipated at the drop of a hat. After her year of being "peepee trained" she finally decided one day to poop on the potty and has been doing so ever since. He will get through it.