He may have some Siamese in him somewhere, their bodies are often long and lean. But they usually have long legs and long faces too. Or British Shorthair or some other breed that has short legs. There is even a 'Munchkin' that has really short legs! Often kittens have somewhat oddly shaped somethings until they get a little older, yours may grow into his torso. I have a little female right now that was hand raised from 2 weeks and she is extremely little, tiny short legs and a peculiar short tail too. I think it had something to do with her rough beginning. Try to get us a picture, I would like to see this cat! Good luck and keep us posted!
First of all, there is no need for concern of him being underweight, you can tell because his ribs are not visible. Some cats are just fine-boned (think distance runner), while others are just naturally more thick-set and muscular (think football player). Secondly, he is not *that* lanky. If you would like to see an example of cats with what is truly considered the "svelte" body type, go to the CFA website and look at pictures of Siamese, Colorpoint Shorthair, Oriental, and Cornish Rex cats. His body type would actually be considered "intermediate", a happy medium between the svelte body of the breeds listed above, and the cobby body type of a Persian or Exotic. The thing is, is that almost 60% of housecats are overweight or obese, so compared to other cats you've seen, a cat at an ideal weight could easily appear to be lanky. Furthermore, he is still a kitten, which means that his metabolism is faster than it will be once he reaches adulthood. Because of this, the amount of food he's eating now to keep himself in this ideal range, will be too much once he reaches 1 year old. I'll provide some links for you to check out:
And here's the "body condition chart", which, I don't rely on the pictures 100% because, again, of a cat's boning, for example a healthy Cornish Rex will look different from a healthy Persian, but what you can rely on is the description of how to feel a cat's ribs to see if s/he is under/overweight.