I've never heard of that antibiotic "cocktail" around here. See, you have different stuff where you are. And, again, I don't know what you're treating, not with any certainty. If he has not been exposed to other birds (he's your only chicken, right? You have no other birds of any kind?), then I'm not sure what he actually can catch. Newcastle is transmitted from other birds. I'm completely at a loss now for what your rooster has going on. Whoever told you he has Newcastle really can't do that without an actual test, BUT Newcastle is caused by a VIRUS, not a bacteria, so antibiotics cannot treat Newcastle. NO antibiotic will treat any virus. Yet, I'm not convinced at all that the rooster ever had Newcastle since it's almost always, from what I've read, fatal.