Frustrated

Will has been in underwear except at nap and night time since beginning of May. He has done excellent with peeing but #2 is about to send me into an early grave. It is miserable. He goes to the bathroom just enough to have to change his underwear at least 5 times a day and he will sit on the potty but will not go. Then it hurts because he has held it in. It is truly miserable and he will finally just poop all in his underwear. I don't know what to do. He would be totally content with wearing pull-ups but I can't go backwards. Rewards or threats are not working either. We took away paci (yeah great parenting since he is 3 years old) and said if he pooped in potty he could have back. Not working so instead he just screams for it and poops on himself! I know this too will pass but I'm hoping it does soon!