It either looks like a bit of tartar, possible staining or maybe a wear mark if he has a thing for biting onto chain link fencing or the bars of his crate. The tooth doesn't look dead, the others that you can see are really nice looking. If you can scratch it off, its likely tartar and otherwise, I wouldn't worry too much. Vets can do fillings on areas that get really worn down, but generally speaking they wouldn't get too worried if it is just a bit of a wear pattern. Their teeth do get a bit worn down depending on how much of and what they like to chew on lol.