Rooster 8 months old w/partial paralysis

I have a rooster (mixed breed, born in April 2015) that we acquired for free in October. We do not know his history or vaccination record, if any. He has always been unsteady on his feet - almost like he is dizzy and does not have good balance especially when he is trying to move quickly. He has always eaten, drank, crowed and pooped just fine so we never were too concerned. However, today when I went out to the coop, I saw him twist his head to one side and fall over. He was not able to get up on his own and his head/neck remained twisted to the left side. We have him with 10 Rhode Island Red hens so I immediately isolated him. He tries unsuccessfully to stand whenever I check on him and his head has remained turned to the side. Our hens have always seemed fine, they were born in June and have never exhibited any unsteadiness. This is our first go around with layers and roosters so I really don't know what I don't know. We have raised broilers so I am familiar with Mareks and did read about AE. If anyone has any information, I would so appreciate it. THANKS!

I'd read here:
https://www.backyardchickens.com/a/the-great-big-giant-mareks-disease-faq

It could be vitamin/mineral deficiencies, but since it has been ongoing for so long, I do suspect Marek's or something that mimics it.

That article gives really good advice on Marek's, and those diseases that can mimic it.

LofMc