These questions will help the experts understand more about what's happening, so they can give you better advice.
1) What type of bird , age and weight (does the chicken seem or feel lighter or thinner than the others.)
2) What is the behavior, exactly.
3) How long has the bird been exhibiting...
@Eggcessive @Wyorp Rock @azygous @casportpony
It could be frostbite, but since you described him as lethargic, his oxygen levels could be low. I tagged some experts, but while we wait, I'll send some of the Emergencies/diseases/injuries and cures questions for you to answer