M.P.
I suggest just backing off and let happen what will happen. It sounds to me that there is a possibility that this has become a control issue and that if it didn't matter so much to you he might decide to do it on his own. Even preschoolers like to have a sense of control and the only thing that they can control is what goes into and out of their body.
Just try not talking to him about it except to say, I know you'll poop in the toilet when you're ready to do it and then don't mention it any more. and see what happens. Try this for 2-3 weeks. He has to trust that it really is up to him.
Along with this don't even ask him to wipe. Let him get used to pooping in the toilet before you teach him how to wipe. One thing at a time.
Many, if not most kids, are not good at wiping themselves at this age. My grandchildren, 9 and 6, still don't consistently wipe. It's not a big deal in the scheme of things. They have so much to learn. Let them learn at their own pace.
Show him lots of approval with all the other things he does well. Let him know, many times/day that he's OK just as he is.
I agree with Sunny M. Holding it is not good for him. Eventually, he'll end up constipated and not able to poop. Let him wear the diapers.