I don't think they have to be pure siamese to have the markings and there are other pointed cat breeds. I have seen litters of kittens on Craigslist that had tabbies, blacks, tortis and "siamese" all in the same litter, with the mom being an ordinary looking shorthair.

But the bottom line is if you tell others he's a Siamese they shouldn't have a reason to say he's not, unless they knew for sure one of his parents wasn't.