Do you know for sure that it is a fear of the potty? Many children have a much harder time pooping on a regular sized toilet than peeing. If they are used to bearing down to poop and suddenly they are expected to perch over a cold space with their feet dangling, it is just too hard.
If that might be the issue you could consider a number of things--- a small potty chair, a footstool and/or one of those padded rings that fit on the toilet.
Is there any chance he is constipated so he is literally afraid of pooping?
Does he stand up to pee? Because I could see having a very different view of sitting to poop versus standing to pee.
For now, I would consider backing off for a few days to a couple weeks and just let him have access to the pull-ups when needed. Then, when he has calmed down a bit (since he is hiding this behavior, I have to assume that at some level at least he is uncomfortable with it) have a talk with him about what exactly is bothering him about the toilet.
Does he use the toilet exclusively elsewhere? Like if he was out for the day what would he do?
Good luck!