What you need to do is stop giving in and letting him have a diaper. He will go eventually, but it might be uncomfortable for him for a while. My daughter is 3 and does not like to poop on the potty either. But, we are firm with her and put her on the potty. She screams and cries, but eventually will poop. We don't yell at her or make a big deal of the fact that she's crying. Either my husband or I will sit near her (across the hall) and let her know that she needs to use the potty. We don't pay attention to the fact that she's crying, but give her encouragement to poop. After she does, we make a HUGE deal of what a big girl she is and reward her with a treat. (We use gummy fruit snacks). It's tough, but eventually your son will be able to use the potty to poop with no problem. Kids will go eventually, I promise!!