Lots of kinds of cats have "tabby" markings, and my cat book says (of the Blue Mackerel Tabby under "American Shorthairs" "The underparts are spotted, and well-marked individuals show a number of darker rings on the neck and upper chest." It also notes that "The mackerel tabby is sometimes described as the "natural" or "wild" type, because this coloring occurs in wild cats."

If his fur doesn't seem healthy, I'd take him to a vet, just in case.