We believe the collar he's wearing is nylon. It seems to be roomy enough, he can shake his head and sometimes he gets it to slide somewhat; and we can fit two fingers between the collar and his neck. We can't see any sores/scratches under his collar when we peek in, nor when he can move it around on his neck. And we're not sure how long he's been wearing a collar. He may be getting used to it, though.

And thanks for the suggestion for the hypoallergenic collar. We'll check it out.