One of the best ways to put weight on a dog is with something called "satin balls". You can find the recipe on many sites on the internet. Some people use it to put weight on their dogs before dog shows, but it's especially good for emaciated dogs and dogs who won't eat. It's high in fat so it puts on weight quickly, but it's also got other ingredients to make it a total diet so it can be fed alone or as a supplement.
My puppy hasn't been eating much and underneath all her fuzz she's skin and bones so I decided to mix up a batch to bring her up to a healthy weight
Step 1: Gather indgredients
This is the original recipe:
10 pounds hamburger meat (the cheapest kind)
1 lg. box of Total cereal
1 lg. box oatmeal
1 jar of wheat germ
1 1/4 cup veg oil
1 1/4 cup of unsulphured molasses
10 raw eggs AND shells
10 envelopes of unflavored gelatin
pinch of salt
However, if you just want to try them out or have a small dog, I recipe this scaled down version (1/10th or the original).
1 pound cheap hamburger (for high fat %)
1 1/3 cups Total cereal
1 1/2 cups uncooked oatmeal
1 raw egg
6 tablespoons wheat germ
1 package Knox unflavored gelatin
2 tablespoons oil
2 tablespoons unsulphured molasses
Pinch of salt
The only thing I suggest you change is the oil; vegtable oil has no health benefit, so substitute it with an oil that does (like flax seed oil which is high in healthy omega 3s).