Hi, and welcome to BYC! Sorry it's on bad circumstances though.
Your rooster might be developing wry neck, or having a neurological problem, or just a vitamin deficiency.
I would get some human Vit E, around 400IU, if you don't have any, and twice a day put one of those in his beak. He'll swallow it. Follow up with some scrambled eggs as that helps him absorb the Vit E.
Also, B-Complex, once a day. That, too, is the human version of B-Complex, and if he's not a bantam breed, you could give him half of one of those per day. If bantam/silkie, give 1/3.
I would also put Poultry Cell in all of their water every other day for a couple of weeks, in case it's a vitamin deficiency and any of your others might be suffering from it but not showing it yet.
Hopefully, this will help him and you should start seeing improvements. I would give him both the E and the B-complex for at least two weeks, but continue with the Poultry Cell.