We have a long, tall and skinny tux-tabby boy who is also commented on for his slender appearance. He's 12lbs, slender, and perfect. Might be the breed? Some are slinky! I was always told that feeling ribs is fine as long as it's when you press gently, during a stroke for example. Neither of mine go outside due to personal choice given the traffic and diseases around my neighborhood. I could not cope with anything happening to them because I decided to let them go explore our crappy neighbourhood.
I second people's comments on the wet food, though, and would add that ingredients are everything, especially where cats are concerned. My 2 boys have been on good quality food their whole lives and at 3 years old, still play like crazed kittens. Keep dry food to a minimum, think "fingerpinch" not "handfull" as well. There is NO such thing as a "dry mouse" and cats are carnivores, they get little to no nutrition from plants and almost zilch from the ground up grains that brands like Hills(?) and Friskies love putting as a main ingredient in their food. Ground Yellow Corn... for example... nothing but industrial waste used to bulk up their slaughterhouse waste, something I would not feed to a pig but people put their hard-earned cash into these products that are worth less than the tin they come in! A lot of the dry food you see out there also has too much filler, and not enough rich protein sources, plus the manufacturing process is said to be appalling. People add water or wet food, allowing stagnant bacteria to flourish. Drinking cannot make up for this dry food diet -- it's not going to help a species known for low-thirst drive and it's not going to prevent those kidneys from working overtime to undo the damage dry food does.
Perhaps if you keep this cat indoors more, you may notice some weight gain? There are plenty of ways to give a cat exercise indoors, a cat tree for example.
Last note, try lightly cooking (or fully cooking) some chicken liver, hearts or even breast for a treat. I leave it a little pink in the middle for my lil guys and they love it! Also, seems basic but I had trouble adjusting as my boys both grew (both are now 12lbs) but make sure that you are feeding appropriate to the weight of your cat, that means nevermind the cans, look at the calories. My boys are on about 300 a day on average if that helps.
p.s. "High grade dry food" is a misnomer. It's actually becoming known as "Kidney disease in a bag". The worst wet food might just be better than the best dry food, with a few exceptions to that rule cough science-diet-"hells"-purinacough.