How old is your rooster? Did the vet say that they did a fecal float for coccidia? Enteritis can affect chickens who have previously had untreated cocci. Enteritis is diagnosed with a gram stain, where most vets only test with a fecal float for worms or cocci. Enteritis can be treated with many different antibiotics.
Including amoxicillin, Tylan, and many others. Metronidazole might treat it , but it normally is used on other things like protozoa infections. I am not a vet, but I would call them Monday, and ask what the diagnosis is. I am thinking (and not a vet) that if it is enteritis, that he should also be treated with Corid and an antibiotic for enteritis either in that order or simultaneously. Feed him a tsp of a good plain yogurt daily with his feed--usually I mix it with egg, water, and a little chicken feed, for probiotics while and after he is on antibiotics.