Perhaps he's not fully grown yet?! My Swordtails are now 8 months old and one of them just decided to become a boy a few days ago! His tail has not even changed shape yet!
That looks just like a swordtail to me. I have a female marigold swordatil that looks just like that, except without the sword. I also have a male platy/swordtail cross, and his body is larger like a platy but he has no sword. Anyway, great fish!
He kinda looks like a female that recently changed into a male. My biggest sword did that,and he looks exactly the same as yours does compared to the others. He's stocky and shorter,while the other males look longer and skinnier.