Staggering, falling over backwards rooster

[Newcastle Chickens]

I have a white Orpington rooster about 8 months old that started limping, then staggering, then falling over backwards about 2 weeks ago. I have given him antibiotics thinking he may have some sort of infection but he doesn't seem to be getting any better. He is eating and drinking normally but he backs up to the pen to keep himself stable. Other than not being able to walk, he seems fine. Does anyone have any idea what this could be? I have 20 young hens too but they are all fine and he is separated from them.

 

[R.M.Qureshi]

How is your chicken Newcastlechickens