My DP's eldest nephew has that same problem. He doesnt have enamel on his teeth so they were always discoloured maybe that could be what your DS has.

DP had the same trouble as a kid they had to put enamel on his teeth every couple of years, hes since neglected to do that and so hes teeth are weak and has had to have a few pulled... wont listen to me and get enamel on them.. nevermind.. his mouth

ETA: He could have knocked it, my mum bumped one of her front teeth a few times as a kid and eventually the nerves died and made her tooth look black/discoloured.