Here's something you can try....
Tell him that he can go in a pull-up, but he has to be standing in the bathroom when he goes. He can be anywhere in the bathroom, but the deal is, he has to be in there.
Once he has that down for several weeks, then change it to he has to sit on the toilet. He can still have the pull-up on, but he has to sit on the toilet. Then you graduate up to no pull-up, and him actually pooping in the toilet.
I know many ADULTS who absolutely will not go poop anywhere but home. I agree that it's disturbing, but it's very common IME.