There are guides to proper body weight, but essentially you should be able to feel a cat's backbone and just feel its ribs.
Your cat probably needs to be wormed, treated for fleas, etc. Almost any outdoor cat will have tapeworms and likely others. One of the topical treatments will take care of the fleas immediately (get Advantage, Revolution, or Frontline--don't use the kind you get at WalMart, such as Hartz--read a little on the internet and you'll know why), and Frontline also treats most worms, but not tapeworms. If the cat is producing classic "tootsie roll" poop, you're in good shape, but if it's very soft, smells particularly bad, or has blood in it, the cat should be checked for internal parasites (giardia, coccidia, TTF).
Many of these things are common to outdoor cats, which is why an outdoor cat rarely lives more than about 5 years. Add in cars, dogs, predators, and cruel people, and you'll understand why indoor cats almost always have a life 3 times that length.
Oh, and the kitten should be tested for "the Big Nasties," Feline Leukemia (FELV) and Feline AIDS (FIV). Both are survivable, but life-shortening.