Interesting I have just taken a look in the search engine and there is much up there about it, go to yahoo and take a look, many links of interest ..
I was born with this "defect" and it has never affected my vision. I'm 18, almost 19. I have been trying to find out what it's called for some time now, but I can't seem to find answers. My eye doctor simply described it as my iris being pulled inward covering part of the pupil, which gives it the pacman shape. She has one other patient with the same thing. It sounds like your son and I have the same exact pupil shape, my mom was worried when I was a baby but it really is just a unique quality that very few people are born with. Your son should learn to embrace it as he gets older, it's a great conversation starter, people are always interested! :)