Could be a few things.
Mareks always comes to mind when a bird becomes lame or paralyzed. Mareks is caused by a virus is progressive in nature. It starts in a wing, a leg, in an organ or in the eyes, slowly paralyzing the bird. Generally attacks young birds, not the older ones. Sometimes you can see Mareks in the eyes. Look into his eyes. You are looking for out of round or oval pupils. (compare his eyes to another bird in your flock that seems well). Mareks is contagious.
He could have gotten into something toxic. This will cause paralysis. There are several good flushes here on BYC that can help to flush toxins and poisons out of the body...http://www.backyardchickens.com/t/700526/flushes-for-aiding-in-toxin-removal
Anything neurological will cause the bird to loose balance, fall or even walk backwards. A bump to the head or even some sort of brain swelling due to bacteria.
This could even be from a lack of vitamins and minerals. Especially the B vitamins, E and Selenium.
For now, definitely keep him inside and warm under a heat lamp. Keep the heat up around 80 to 85 degrees Fahrenheit. Sometimes birds go off their feed when they are sick. So feed him what EVER he will eat right now. Hard boiled eggs are good emergency feed for all birds. They contain all the building blocks of life including those I listed above that he might be lacking. Make sure he is drinking as well.
He may recuperate in a couple of days. You might also try one of these flushes too. If he continues to go down hill, you may have Mareks in your flock. Have you had them vacinated for Mareks?
Keep us posted and I do hope he makes a full recovery!! :-)