We went through the same thing with my nephew when he was potty training. He refused to poop on the potty and would hold it for days, finally ending up having accidents in his underwear. After speaking with his pedi, we began giving him Miralax every day to get things moving again, and when we would see him start doing the "poo poo dance" (as it became afectionately known as), we would sit him on the toilet. He also likes the cherry-favored prunes, so we gave those to him for dessert sometimes. We would reward him when he went on the potty, and eventually he started going on his own. It did take a few months and a lot of patience and dry runs, but he eventually got comfortable with it. He has been accident free for a year now. Just stick with it, and your son will eventually lose that fear of going on the potty, too. Hang in there...it will get better!