hmm, are you raising a Sheltie or a market calf?

I kid, I kid!
How are you figuring that he's underweight?
We often think a dog is "just right" when they're actually overweight, and think they're emaciated when they're at the correct weight. You will do the dog more harm keeping him a little over than a little under.
You say you can feel his ribs easily - that's a good thing! A dog's sides should feel like the back of your hand: you can feel the bones easily, and there's a thin layer of flesh over them. Too skinny, and they'll feel like the backs of your fingers. If the dog's sides feel like your palm - if you have to press to feel the bones - the dog is too fat.
Plus, this is a one-year-old dog - he's a teenager! His metabolism is zippy, he's an active dog, and he'll naturally start to fill out as he matures. He looks like a gawky beanpole right now, but that's because he's still a kid. (Think of a high school cross-country runner.)
My advice is to be patient. Unless he's losing weight or truly emaciated (which seems unlikely) he's just maturing at his own pace.