If you are going to take responsibility for this cat then taking it to the vet would be a good idea. If he has been living out doors then he may have worms or other parasite which will impede his recovery.
Since he is not drinking water, you'll want to go with a canned food to make sure he's getting lots of moisture - dry food will further dehydrate him.
I don't know what kind of foods are available in your area, but look for a high quality, grain free food where meat is the top ingredient (it might be the second one, after broth or water sufficient for processing, but you want to avoid foods that are primarily made from meals, digests or by-products.) Innova Evo and Orijen both make grain-free cat foods.
Kitten foods tend to be higher in fat and protein than adult cat foods so you might want to choose a kitten food to help with the weight gain.