While you're seeing some improvement in Elvis, don't trust it. Birds are among the very best "maskers" in the animal kingdom. No matter what and no matter when or in front of who, they will pretend to be just fine until they can't hold the act any longer. By then, whatever the problem is, has advanced. Sometimes beyond help
As for what might be wrong with Elvis, given that you note normal droppings even now, fortunately blockage or impaction is unlikely
If he's truly male and you know this beyond any doubt, we can rule out egg binding which often presents with these symptoms
Unfortunately, beyond that these symptoms often accompany a variety of health issues and only upon a hands on exam can we figure out which one it is
Birds don't usually hurt their backs or legs playing normally in their cage so breaks and sprains are low on the list
If Elvis is a seed eater we may be looking at liver disease (hepatic lipidosis most common) or even a tumor somewhere inside
Getting him immediate veterinary care is essential. Remember, do not trust what might look like recovery no matter what
To help him survive until you get that care:
Use a thick, clean sock and fill it ¾ with plain, raw white rice. Knot the end and microwave it for about 1 ½ minutes. Shake it afterwards to distribute the heat and be sure it's not too hot. Tuck this in just under the cloths.
A heating pad under the box is also helpful, set on low. This is one of the few times I’d ever use both heat sources if necessary to maintain incubation temp (90-105 degrees).
If ever using an electric source for heating anything in anyway, please be vigilant and constantly double checking carefully.
Gently drape a light cover over this box to further help hold heat in and keep light low.
If he seems to be fading, try some corn syrup, maple syrup, honey or plain sugar water (tablespoon real sugar in 1/2 c of water) to the side of his beak so that a drop gets inside (do not force it down his throat or anything)
If you have a Pet Smart near you they generally have a Banfield clinic inside and they treat small birds like this; also, call Pet Co and every other pet store and vet in the area and ask who they use for birds. There are probably more than you think
Good luck with him and please let me know what happens ok?