my toddler won't poop on the potty she holds it for 4-5 days help!

My son is almost 3 and we are working on potty training. He also does not like to poop on the potty. He will hold it, sometimes a few days, until he can really hide and do it. We have tried bribes and encouraging him but he just wont do it on the potty. We also explained to him that his tummy hurt because the poop needed to come out but it doesnt seem to work. I try not to push him because I dont want him to get scared but I dont know what else to do. All I can suggest is to make it seem fun and bring some books into the bathroom.That helped us a little bit. Good luck