One possibility is that he is just built that way -- I have a cat who was a total string bean for a really long time. Now he is 5 and is a fatty. It is not a bad thing when a kitty is good at weight maintenance! Much harder to put them on a diet.
Another possibility, especially if his coat feels greasy or dry, is that he might have worms. Have you taken a stool sample in to the vet lately? There are over-the-counter worm preparations, but I don't recommend them -- they usually only cover one kind of worm, and it's one rarely seen in adult cats, and it's never a good idea to dose your cat randomly for things. If you think his coat doesn't look as good as it used to, take a stool sample to the vet and ask that Mr. Slender be checked for tapeworms.