I was born with no fingers on my left hand. In terms of getting on with the usual stuff of life it's no problem, but a few years ago I began IM correspondence with somebody who, after a few weeks, began to talk about us meeting up. I then happened to mention my hand... and she began to lambast me for not telling her about it before, as if it was some hugely important thing. It did show her up as shallow, but it also made me feel incredibly self-conscious. Even now I walk around with my hands in my pockets far more than I used to - that sort of reaction can make you feel like s**t.
All I can say is try to understand why he may not have mentioned it before. Maybe it has - unreasonably - been a barrier to previous relationships and he just feels very reluctant to mention it. He may be far more normal than you think.