Welcome to BYC,
I can't really see what is wrong with the rooster from the picture you posted, maybe you want to visit a veterinarian that lives close by or you can wait for a while and see if the rooster gets any better. The rooster might have a disease so you might want to isolate him from the rest of the flock and check your other chickens to see if they are behaving strangely.
Are there any small wounds on him? If so, it could be that he was picked on by the other chickens... If there was already a rooster in the flock, the new rooster would have ended up in a fight with the other rooster since they are very aggressive.
Good Luck and I hope your rooster gets better!
Julia