You should probably talk to a vet to find out why the dog is malnourished, there are some things like parasites that can cause it and the dog would need to be treated for those before it could get better. If there is no underlying condition causing the dog to be malnourished then feed it a good quality dog food, there are usually instructions on the back of the bag for the amount you should feed him but you might want to increase the amount until he reaches a healthier weight.
Buy dog food and check the amount you should feed it according to its weight. Then divide that food into five portions and give it one portion after every half an hour. If it does not change after 2 to 3 days you can get a high-quality brand of dog food. If all this do not work you can take it to a vet. See more information at http://www.ehow.com/how_7312241_care-malnourished-dogs.html.