Excellent advice above! Definitely follow the advice from
@2ndTink on giving Vitamins, Vitamins never hurt and deficiencies can definitely cause weakness, wry neck and ambulatory issues. The B Vitamins, especially B1 along with E and Selenium are very important for muscle coordination. It can take weeks for the body to respond to added vitamins if there is a deficiency.
I've never a chicken walk backwards however I have had quail do this after a head injury from flying into ceilings, walls, ect... If this is the case it can take a while for swelling in the neck and brain to go down. I would definitely keep him inside somewhere warm (he will heal faster) and keep him in a small space until he recovers, he shouldn't be allowed to back into things and injure himself. One baby aspirin once a day will help with inflammation. (No matter what the cause, Id keep him inside, warm and confined anyway.)
Toxins in grains can cause walking issues as well as can an inner ear or encephalitis (brain swelling) infection. If after you have ruled out all other advice, you might consider toxins or bacterial infection. These are less likely but are always a possibility.