Do you have photos of the material inside the beak?
If there's a bad rotten odor, then usually that's Canker, so you're correct, Metronidazole would be the treatment of choice. Or you could use Acidified Copper Sulfate.
If there's no bad odor, it could be Wet Form of Fowl Pox which can cause lesions inside the beaks, but generally it affects the eyes with scabby lesions as well. An antibiotic may help with secondary infection when dealing with Wet Form of Fowl Pox.
If you're detecting a sour or yeasty odor, then it may be sour crop or yeast infection. An antifungal like Miconazole, Fluconazole, Clotrimazole, Nystatin can be used for treatment.
For the eye, if it's due to fighting, then flush/clean with saline, remove pus and apply Terramycin in the eye, do this a couple of times a day until healed up.
I've had roosters fight many times and have injured eyes, broken beaks/toes, etc., but never dealt with them developing any type of lesions inside the beak. The best I can tell you is to continue to address symptoms and make sure he's eating/drinking well.