skin and bones dachshund can't gain weight

[acerhound]

I adopted a  4yr old male dachshund a couple months ago who appears healthy and has had all his routine tests and shots and everything appears normal. He eats with great gusto (surprise! he's a dachshund) and has plenty of energy. MY worry is his weight. He's a 'tweenie' size but only 12.5 lbs -- clearly too thin. His vertebrae and rib cage are prominent despite giving him two meals a day and frequent snacks. My efforts to fatten him up have only served to cause my OTHER male dachsie (14 yr old) to gain weight, since I give them both treats (the older guy gets much less quantity)! He remains after 2 months 12.5 lbs. After I took him home at an initial vet visit (just to introduce and get a precautionary stool sample), they said they don't see anything apparent to worry about. Only other sign I see is swollen glands under his ears at the mandible, which the vet said didn't seem worried about. I don't want to get into the costly and fruitless merry-go-round of tests that vets can put you through, but I fear something is wrong. Any guidance?

[ginger899]

One question...has he been neutered? Sometimes, 'entire' male dogs manage to burn up an awful lot of nervous energy, and can remain quite thin, despite being well fed.

Another couple of points....a test for hyperthyroidism (though it's not so common in dogs, so I hear) -might be useful, also urine testing for diabetes.

Does he get more exercise than your older dog? A very energetic young healthy dog can burn off energy quite quickly, and if he gets a lot of exercise (even running up and down your yard could do it!) -he may need more food to compensate. Same in cold weather.
The recommended feeding guideline, based on a dog's size and weight, can vary if: the dog is a 'working dog'...if the dog gets a lot of exercise...if it is cold weather. (And usually, in hot weather, dogs need a little less, though I know that's not the case with your dog.)

Another small point. When you feed them, do you feed them together? Do you see who eats what? Or do you walk away when they are eating? Sometimes a 'greedier' dog will snaffle part of the other one's food! I have caught my dog doing that if she is fed alongside another dog.

[have 2 kids]

How much are you feeding him at a time?