J.,
My son did the same thing...would go just fine (tinkle) in the potty, but not the poop. And, he had gone once, so I knew he could do it. I know they say that it can be "scary" for them to go poop, but this is what I did and I swear, I felt like a heel for doing it but within 1 weeks time (I think really 3-4 times of pooping in his pants) he was trained.
** I would ask and take him regularly to the potty. If he pooped in his pants, I would go into the bathroom w/him and have HIM rinse them out in the toilet (of course they can't do it all, but it's really just the fact that they have to do it at all). He looked up at me with his sad face and said I don't want to do this and I calmly said well, neither do I and until you can go like a big boy, you will be rinsing out your undies. It didn't take long!
I also rewarded him and gave him a geotrax toy that he wanted really bad....but I had also told him that if he pooped his pants, that I would take it back...he could only keep it if he was a "big" boy.
Best of luck!!
37 y.o. mom of 3 boys 11,9, and 4. Married 13+ years and work full time in healthcare and sell southern living at home for fun!