I am also having issue with a 1 1/2 year old rooster now cannot walk.
It has been a 3 or 4 weeks now. ( I just can't put him down

)
I have 20 chickens total, 5 are roosters. Everyone is fine but him. Started one day i noticed him limping. Thought maybe he was fighting and got hurt. Then after a couple days he was having a hard time walking. Next day he was just walking like a drunken sailor, no balance, kept falling over, face planting. So i caught him and put him in a small cage in a nest box with food and water right in front of him. He kept just flailing around trying to walk and couldn't.
Also his comb is droopy (Like a killer whale in captivity) and pale, no longer bright red and standing upright.
I have started giving him vitamin b-12 every other day and got some Rooster Booster to put in his food. And giving him some treats (Chicken crack from treatsforchickens.com) just to keep him interested.
All chickens eat layer pellets, I throw scratch on the ground each morning and they free range all day every day, and I throw watermelon or berries to them on occasion as well.
All other birds (roosters included) are doing just fine.
I have no clue what is wrong with him! Please help!
Here is a pic of what he looked like a few weeks ago.