Have Bird Questions? Ask a Bird Specialist.
I am sorry to hear you are worried about your Mockingbird. My name is Rebecca. I am a veterinarian and will do my best to help.
How long have you had him? What have you been feeding him?
The most common reason for hand raised birds to develop limping, weakness in the legs, and then not to be able to walk is metabolic bone disease caused by a diet with too little calcium and phosphorus. The bones as they grow become weak, and rubbery. Not being able to walk is often the first thing noticed.
Good that there is crushed eggshell in your food. This is less likely to be metabolic bone disease then.
Still, it has to be on the list because the symptoms are what you see. Other possibilities are parasites, or some neurologic infection/condition.
Sick birds are fragile and hard to treat. Birds like mockingbirds are federally protected; the only ones legally allowed to care for and treat them are licensed bird rehab specialists. I am a veterinarian, and cannot legally treat them; I have to send them to my local wild bird rescue. Contacting your local wild bird rescue and having the experts there check him out would be best. Where do you live? I can search for the closest bird rehabilitator in your area.
How close is Greeneville? http://www.wildlife-rehab.com/
This is the list for all of South Carolina: