My cat is very underweight?

  My cat Rico has always been on the skinny size, but ever since we got my new puppy, he has lost weight, and is now only 5lbs. He gained 1/8 of a pound back when we fed him kitten food (the vet recommended this because kitten food is higher in fat), but it has pretty much plateaued from there.
Lock you cat in one room with the litter box for 24 hours so that you know the poo belongs to him and get it tested. Raw food diet is great, but not to be taken as an "easy fix" as it can easily become unbalanced in the nutrients. You have to know what your are doing in feeding a raw food diet. Some cats do not immediately take to it either. Have you tried wet food, cat treats? If you are really serious about the best nutrition for cats the raw food diet is the best, but you have to educate yourself first and it takes prep time. Here is a link with good recipies and the benefits of raw food diet. And yes, your puppy has thrown his world upsidedown and your cat is reacting to it. Try to give him a quiet room to himself where he feels safe and calm. pets question and answers,www.5d2d.com

http://www.romanbengals.com/rawfood.html
This sounds like your kitty is very stressed about the puppy's arrival. He probably isn't eating because of the puppy. He is scared and stressed and burning a lot of fat! That being said, you may want to have your veterinarian run a blood test to make sure everything is alright with him.He could have an underlying health problem that has come about due to the stress of the puppy.



Kitten food is an excellent idea! You may also consider canned food and raw food. Also, make sure you give your cat a safe place to go where the puppy isn't allowed. If you know your kitty's favorite hiding place, block that area off from the puppy and put his food and water in there. A baby gate is an excellent way to keep puppies out of certain rooms but still gives your cat access.
First I would have him checked by a vet for health problems. Many thing s can make them lose weight. (kidneys, heart, infection) He might be stressed out about the puppy as well make sure he has a place to get away from the dog and make sure you set boundaries for your puppy, no jacking with the cat, that's the rules at our house.
Try some canned food and mix some appetite stimulant in. It comes in a tube, like hairball jelly. Nutri-cal is one brand. If you need a poop sample, lock that cat in a room with his own litter box until he provides one. I suspect it is just the stress of the new pup though. Make sure his food is located in a quiet spot well away from the pup so he can feel comfortable eating. Good luck!
feed him raw meat dammit
maybe its worms?