J.N.
I'm no expert, but this definitely sounds like a sensory issue to me. My own son is shortly going to undergo a sensory motor integration test, but since we haven't had it yet I can't give you any more help in this area.
However, Children's Hospital has a toilet training program (I think it's called "PIP") for kids who have physical or emotional problems that prevent them learning to stay clean and dry. I was investigating it for my son but all of a sudden he started making real progress so we haven't gone ahead with it. But the woman to talk to is Kimberly Dunn, if you're interested. I don't have a number but I'm sure you could call Children's general information and ask about the PIP clinic.
Good luck!
J.