Read him some books about potty, and teach him that food goes in, our bodies take the nutrients they need to help us grow strong, and then what the body doesn't need comes out as poop. It might help if he understands why.
I know it sounds gross, but maybe letting him accompany you to the toilet to show him that when you poop in the toilet your bum is clean might help him decide that the potty is the way to go.
Don't wrestle him down and force a diaper change. When he screams about it, comfort him and ask him what he would like you to do. If the diaper change is his idea he will be a lot more likely to cooperate. If he chooses to stay in it, let him be and just occasionally ask him if he would be more comfortable with a clean diaper.
Be sure that his poop is not too firm and hurting him. If that is the case, then get that resolved before he starts holding it in... That is a bigger problem.
If you think he is ready, and his poop is solid (not baby paste), then leave him naked with a potty near by and just keep a close eye on him. Not having the diaper or underwear to mess in may force him to choose the potty.