Underweight Dog

Question
Hello, I have a 4 year old, male shitzu and poodle mix dog that just got neutered. When I picked him from being neutered, the vet said that he was very skinny. I have taken him to his regular vet, and they said he was skinny for his age and breed. They tested him for worms, and tape worms and everything has came back fine. They told me to give him a high calorie food, but did not give me examples of what a high calorie food is. I gave him a high protein food, but he is very picky with what he eats. He will only eat at night when my husband and I are in bed, and will only pick at his food. We have tried canned dog food, and that works for a little while, but then he stops eating that too. He is up to date on all shots and acts completly fine. His personality, and energy level have not changed. He does sleep alot, but that's the only thing that he seems to do differently. What can I do to get him back to a healthy weight? He has been skinny since a puppy, but now you can see his ribs and his hip bones. Please help. Thank you in advance. By the way, we have 3 other dogs, could this be one of the issues with him only wanting to eat at night when the others are put up?

Answer
Frankly a healthy dog that won't eat all it needs is quite rare.  You might even have another vet have a look.  Usually a dog will eat better around other dogs competing for the food.  

The less grain and more meat in a dog food, the higher calorie the food.  Also, in many cases, the higher the price.  You will pay a high price for Orijen.  For most dogs, it is of no benefit.  You can go to their website and check its calories.  You may also want to look at some other things, Flint River, Taste of the Wild, California Gold, and others.  When it comes to dog food, there is no limit on hype.  Many of the more expensive foods are marketed through small reps some of whom provide biased advice over the internet.  Oh and the lady here that sells NUVet suppliments will suggest them.  They are worthless.  The help at pet supplies often push foods with their best mark up.  

Even once you identify a higher calorie food, or even try a few, you may find he eats less to continue to keep himself thin.  We did switch to a higher calorie food for our one Lab, but he still wouldn't eat enough to completely hide his ribs.