We had the same issue and I knew that I was not going to buy diapers or pull ups for her to poop. We decided to ditch the diapers for good during memorial day weekend, so we had an extra day to practice before it was back to work/daycare.
I planned carefully and were out of diapers at that point and I made it clear that I was not buying new ones. She also had plenty of advance warning during which we talked about the plan.
I made sure to feed her lots of fiber and stool softening food (fresh and dried berries do the trick for us), so going was easy.
When she had to go and asked for a diaper I sat down in the bathroom with her and encouraged her to go. I also rewarded her with a treat for every bowel movement for the first week or so.
Yes, there were a few accidents. Mostly during the first week, now we are three weeks in and down to maybe one accident a week, mostly at daycare. We don't make a big deal about it, neither do her teachers (usually happens when they are playing outside and they forget to put her on the potty before they leave). I still give her an occasional reward, especially in situations where she doesn't want to interrupt her play - but then does to go.
I too, had to remind myself not to get aggravated with her, especially if she had an accident after I had just asked her to go potty and she said she didn't need to. But if you want to be successful you MUST keep your cool during those first challenging days and weeks.
Rather than impressing on him how much of a pain it is for you, try positive rewards and encourage him.
The few times were I showed my disappointment (involuntarily) I really noticed how my daughter's confidence shrunk and a pee accident was sure to follow too or she wanted to go back to diapers even more.
So I would say, if you can commit to keeping your cool and cleaning up a few poopy pants... just give cold turkey a try.
Good luck!