I like Lois response too.
My daughter was born with a port wine birthmark on her forehead and we heard all kinds of questions which I always felt were rude as I would NEVER ask a stranger why their child had that mark on her head. One woman even asked if we had burned her with a cigerette!!!
I finally started telling people "Oh, that's where the angels kissed her when they sent her to us". It shut people up and my daughter who was beginning to understand people and the world around it loved it and for years would tell people it was her "angel kiss" and never felt self conscious about it. In fact, I heard from a couple of her friends mom's that they wished they had been kissed by the angels too.
I think it's all in the way you present the answer. And Lois' answer is a good one.
As for the "normal" questions, I know I'd have to say "what in the @#$#@ makes you think he ISN'T normal???"