While it sounds ridiculous, my 3yo is terrified of wearing underwear. To the point where he will kick and scream and even if you get them on him, will keep screaming until they are off. Even if they're on over a nappy. Even if we promise him a treat if he keeps them on.
He's doing fairly well with training itself (though the hot weather has meant he considers his paddling pool to be an outdoor toilet! 
) but how can I get him out of nappies if he won't wear pants? I have no idea where this fear has come from, he hasn't had any traumatic pants-related experiences that I'm aware of.
Could it be a "new clothes" issue rather than a "pants" issue? Though he's normally okay with new t-shirts and trousers, could it be he doesn't like the change?
Any suggestions are welcome, I am stumped!