I do not see an emaciated horse there, just a young, growing Walker gelding. Gaited horses, especially Walkers, DO go through odd growth spurts and look "gangly" well into their 4 and 5 yr old year. They should not be round and filled out like a stock horse. Walkers are supposed to be angular and "sleek", more like a racing Thoroughbred, with long and lean muscle.
Your boy looks like he just needs more riding to build muscle, and maybe add some alfalfa (hay or pellets) to help keep his weight up with increased activity. Adding some flax or corn oil to his feed wouldn't hurt either, but really, he's in fine condition. He's no where near "emaciated".
I don't see any ribs, just water marks or shadows. At his age, you SHOULD see rib when he turns or stretches out at a gallop. If you see rib in person, maybe you could get some pictures of him at different angles when he's completely dry.
I would have the vet out to check his teeth, just in case. I'd also deworm him now with Pyrantel paste and then Ivermectin in 4 weeks. Then keep him on a good deworming schedule. That will make sure he's parasite free and able to absorb the nutrients he's eating.
He's a very pretty boy!