One of the little boys I watch in my daycare had a simliar situation. When he went in for his 3yo well check, the ped told mom not to give him diapers anymore for pooping. If he knows he needs to go enough to ask for a diaper, he can go in the potty. This little boy held it and held it and got constipated. Dr. told them to take Miralax once a day. It is the most gentle laxitive out there. This helped since, at this point, he held his poo for over a week. He had one day at home where it was quite loose, but he never had an accident. It wasn't hard (constipated) so it didn't hurt and it was a little bit often, which gave him lots of practice. He got comfortable with it and now has no problem.
Having heard about their experience, I felt a little prepared when my son was potty trained a few months later. He also was nervous about going poop in the potty. A few times he would save it for nap or bedtime (when he was still in diapers). Sometimes he would wait a couple days before finally going. We refused to put him in a diaper for pooping. We just believe if he is old enough to ask for a diaper for that, he is old enough to go in the potty. Sometimes we had to tell him we weren't going to ____ (park, store, dinner, etc..) until he tried. We had him try for 3 minutes (because he was 3yo) with privacy. It usually worked pretty quickly. He always goes better if no one is in there with him. I tried staying in there and reading books to him or just keeping him company, but he never went. The minute we left the room, he relaxed enough to go. So we tell him he has to try for 3 minutes with privacy. :o)
We also explained the whole digestive process (in preschool terms) to our son. This really helped him understand why the poop had to come out. We explained what happens if it stays in there and what constipation is. It all made sense to him.
Just be consistent. Whenever he is showing signs of needing to go, make him try for 3 min (or whatever). It is a lot of work...just like starting potty training over again. He will eventually go. Possibly try the miralax if constipation is an issue - check with your doctor first. Don't give a diaper to him. Talk to him about why he doesn't need it anymore. He is a big boy and he can do it. He's done it before! Good luck, Mama!