Hi,
This is totally normal. Many kids have "poop phobias"--boys and girls. If he asks for a diaper or pull-up to poop--just give it to him. Eventually he'll just go on the toilet--and it won't take as long as you think.
Our son would wait to poop until "quiet time" when he was wearing his pull-up...then one day, he had to poop in the morning--he got all red in the face and my husband and I just took him into the bathroom and camped in there with him because we could tell that he had to go so badly he couldn't keep it in. He cried, but we sang songs and encouraged him and he did it! We took him for ice cream and he began to poop on the potty after that (we had to watch for the signs for a while and then take him to the potty and sit with him there).
Don't push this too hard, though, because you can make your kid constipated and then pooping will really be a problem because it will hurt!
You can try feeding your son some high-fiber foods, too, and try to catch him in the act while being very encouraging.
:-) D.
P.S. I bought "potty toppers" for public toilets--this really, really helped our toileting fears when we went out. Kandoo wipes are fabulous, too.