Before starting any kind of diet change, take our Chihuahua to the vet and find out if there are underlying causes to why he isn't a healthy weight- or he could BE a healthy weight, he just look skinny (I have an 60lb dog that people think we starve even though he is perfectly healthy). If everything checks out, the vet will probably recommend a prescription diet for him which will help him put on the weight gradually and won't make him too big, too soon which can cause health problems.
Have you ruled out any medical conditions?
If you have, then you could try a paste like Nutrical, which is a high calorie paste used to aid dogs that either can't eat for whatever reason or dogs that need to gain weight.
Read the dosage directions on the package and even then with a small dog I would start with half that. If you don't see a change, add a little more.
After you have put some weight on him, you will probably need to feed him more each day than you do right now to maintain it. If he really can't maintain his weight on an appropriate portion of Wellness then its time to switch foods.
Even though Wellness is a good food, it isn't perfect for all dogs, also, there are other great foods out there.
awwww! maybe its time to feed him through his ***
Google satin balls and follow the recipe. I fattened up a dog within two days feeding it and it was made of ingredients from my kitchen.
Buy your dog a brand of puppy food. Puppy food is packed full of fat meant to make a dog, well plump. Remember when changing a dogs food to slowly mix the new food with the old food to phase the old food out of his diet.
Try feeding him puppy food. It has a higher fat content.
get him stuffed