How to Get Dogs to Gain a Healthy Weight
To get dogs to gain a healthy amount of weight, visit your veterinarian to ensure there are not any underlying health concerns that need addressed. Sometimes, intestinal worms or illness are the cause of weight loss in dogs. Ask your vet what the ideal weight is for your dog is and what food they suggest. To increase your dog's caloric intake, add another meal to its diet. You can also supplement your dog's regular food with some dog-safe human food, like hard boiled eggs or plain sardines.
Record your dog’s weight. If you believe that your dog is underweight, you need to begin keeping track of the dog's weight so that you can track the dog's weight loss, and then his weight gain as you implement weight-gaining strategies. Be sure to share this information with your vet. Consult a veterinarian. You need to be sure that there is nothing causing your dog's weight loss that needs medical attention. Your dog may have an illness or parasite that isn't immediately obvious to you that your vet will be able to determine. Illnesses such as diabetes, cancer, hepatitis and inflammatory bowel disease may manifest as weight lost, but require additional treatments and medications. Therefore, a pet with an undiagnosed condition will not improve with food alone. In fact, they will get much worse without proper treatment. Determine your dog’s ideal weight. Talk to your vet about using body conditioning scoring (BCS) to help objectively assess whether your pet is too thin, too heavy or just right. A sample chart can be seen here. If your dog seems underweight based on the BCS, you should talk to your vet about an approach to help your dog gain weight. In general, your dog is at a healthy weight if you can see their waist, you can feel their ribs when petting their sides without actually seeing the outline of the ribs and their belly curves upward to meet their hips. If you can easily see most or all of the ribs, the backbone or hip bones, your dog is likely too thin. Some breeds, like Greyhounds, and some hunting and herding dogs (Border Collies, Pointers) tend to be leaner than other breeds like Mastiffs and Labrador Retrievers. Deworm your pet. It is best to have your vet do a fecal test for intestinal parasites, but if you are unable to do that, you can diagnose and treat your dog for worms from home. A dog that has an intestinal parasite infection may be underweight because the parasite is sapping the nutrition out of the dog's food before the dog is able to process it. Ensure that your dog is getting appropriate exercise. A dog's weight is related to his overall health, and part of your dog's health is getting adequate gentle exercise. Make sure to consult your vet before embarking on a rigorous exercise plan. Some dogs suffer from arthritis, neurological difficulties or metabolic diseases that lead to muscle wasting which would require special treatment under the guidance of your veterinarian to improve your dog's health without additional injury. Gradually increasing controlled walks on leash is usually safe for most dogs and less likely to cause injury. Swimming is a great no impact exercise for dogs who don't mind getting wet. Make sure the entry and exit from the water are set up to prevent injury getting in and out of the pool, lake, or river. Add an extra meal into your dog's daily diet. If you feed your dog once a day, add a second meal. If your dog is already fed morning and evening, add a meal during the middle of the day. Do not necessarily change the diet you are already feeding, simply add one more meal to the day to add calories.  Keep in mind that by increasing your dog's diet by a whole meal, you are also changing the bathroom habits of your dog, which may require altering your dog walking schedule. Assess the quality of your dog food. Dog food comes in a wide variety of quality. You should be sure that the food you are feeding your dog is providing an adequate number of calories and an appropriate balance of nutrition. Checking the protein and fat content of the food you currently feed is easily done by checking the information provided on the side of the bag. The calories per cup will not always be found on the package, so you may have to go to the manufacturer's website or call the manufacturer to get that information. You will also find an ingredient list on the side of the bag. Look for foods that begin the ingredient list with a protein, such as “beef” or “chicken” or “lamb” instead of a carbohydrate like corn or wheat. You can check the quality of ingredients of your current dog food http://www.dogfoodadvisor.com/ here. Talk to your veterinarian regarding your pet's nutritional needs including the recommended caloric intake per day for your dog. Additional information can be found at: http://vet.osu.edu/vmc/basic-calorie-calculator. Supplement the dog’s food with appropriate human food. Adding some yummy, safe “human” food to your dog's diet can help increase a dog's enthusiasm for eating. Non-fat, no salt chicken broth, beef broth or vegetable broth is very tasty, especially if you warm it up before placing on top of the dog's food. You can find non-fat no salt broths in the grocery store pretty inexpensively. A few tablespoons will add nice flavor to your dog's food. A few tablespoons of roasted chicken without the skin, boiled egg or plain sardines (or mackerel) will often help awaken the appetite of most dogs and are healthy sources of protein and calories. Dogs can get sick if fed too much fat, so stick to protein and carbohydrates to add healthy calories to the diet. Other foods to try are the water from canned tuna, non-fat cottage cheese, non-fat plain yogurt, or plain canned pumpkin. Avoid human foods that can be dangerous to dogs' health, such as chocolate, raisins, grapes, onions, garlic, and anything with mold on it. Try a different kind of food. If your dog is not responding well to the food he is eating, try giving him a different (high quality) dry food, a high-quality canned (wet) food, or follow a respected homemade dog food diet. High quality pet foods generally list the first ingredient in their formulations as a protein, such as “beef” or “chicken.” If you are home cooking the whole diet for a dog over a long period of time, it is extremely important that the diet is complete and balanced nutritionally. Using a reputable source from your veterinarian for your dog food recipes is recommended, and make sure that you don't leave out any ingredients when you cook. No one diet is “perfect” for every dog, so be sure to do some research with the help of your veterinarian before changing to a home-cooked only diet for your pet. A few resources to consider when starting your research are The Whole Pet Diet by Andi Brown and the Dr. Becker's Real Food for Healthy Dogs and Cats by Beth Taylor. Add water to your dog’s dry food. If your dog is not enthusiastic about eating his dry food, try adding hot water to the dried food then letting it cool so that it's mushy. This often makes the food more appealing to dogs. 
Record your dog’s weight. Consult a veterinarian. Determine your dog’s ideal weight. Deworm your pet. Ensure that your dog is getting appropriate exercise. Add an extra meal into your dog's daily diet. Assess the quality of your dog food. Supplement the dog’s food with appropriate human food. Try a different kind of food. Add water to your dog’s dry food. 