Pets, like people, come in different shapes and sizes, Alexis. It’s possible that your dog is slight in build and is meant to be that way, especially if you’re feeding him a high-quality, nutritionally complete diet and your veterinarian believes he’s in good health. If you think a health issue is causing his low weight, you should speak to your vet in detail about it.
You can also check the feeding guidelines on the food you feed him. These can give you some indication as to the amount of food your pet should be eating. Just keep in mind, individual pets may require slightly more or less food than is indicated in the guidelines. We would also recommend discussing your pet's appetite with your veterinarian. She’ll determine the appropriate amount of food your dog should eat, then compare that with the amount of food he does consume and, if necessary, provide recommendations to increase his dietary intake.
Remember that a high-quality, nutritionally complete diet is important because it makes nutrients more available to your pet, allowing him to lose less nutrition through his stools.
More guidelines on healthy canine eating habits are available at our Top 10 Tips for Feeding Your Adult Dog and How to Visually Assess Cat and Dog Body Condition.