One thing that hasn't been mentioned as a possibility as to why this chick appears to be stunted in its growth is that it has failure-to-thrive syndrome.
This basically is a chick that didn't have a full set of functioning equipment at hatch, and this hobbles it as it tries to process the nutrients in the food it eats, resulting in a very inefficient metabolism. These chicks don't keep up with the normal pace of chick growth, and seem to develop other secondary issues as well.
One of the issues is they seem to crave a heat source longer than the other chicks, having trouble staying warm, and they may feather out slowly, if at all. They are sometimes troubled with constipation and impacted crop. (rarely sour crop). This is often what kills them.
The things you can do for this chick are to get additional nutrients into its system (Poultry Nutri-drench) and try to alleviate it's constipation/crop issues with coconut oil, about one teaspoon should do it. The crop can be impacted and still feel soft, and if it has impacted crop, it likely is constipated, too. Offer plenty of water and only minced boiled egg or yogurt for a couple of days.
Some failure-to-thrive chicks make it, and grow up, but they may never be full size for their breed. They may never have a fully mature set of organs, and this can lead to a short life. Many are picked off by predators because they aren't as able to evade and flee as normal chickens are. He's made it this far, though, so he has every chance of making it with a little boost.