I could suggest that maybe he is being sexually abused but that would
be sadistic on my part considering the way I feel about the whole
thing.

Talk to the child about it calmly and quitly than maybe your son could
tell you why he dose not go poop in the potty. There have been some
strange stories such as the child coming to believe they are losing a
part of themself like some usefull part of their body is falling into
the potty or other such things. Also if you can't get into his head
try a shrink they may be able to help.