As long as he's not seriously thin, ie: you can't see any obvious bones, then it's probably nothing. Male cats do lose a bit of their muscles when they're neutered, it's most evident around the jaw, take a look at any un-neutered male and they have heads like boxers. It's probably to do with the fact that they no longer have testosterone pumping up their muscles. I would think that this is normal and if your cat is acting fine then i presume that all is well