Are you sure it is not the water? Does the fish feed?
Some antibiotics do not survive the acidity in the stomach, so, as you well know, are given e.g. to humans via an injection or to fish via water=absorption through gills and skin. The med's label should state if it can be administered via food or one can search on the net and/or call the med's manufacturer.
IDK how and why but some here report having cured cloudy eye with Melafix.
I hope it is not the beginning of hemorragic septicemia. If it was, I think a penicillin-family drug would be needed, like the erythromycin you mentioned.
Any sense in giving the fish a good, through-food antiparasitic treatment and keep the water as pristine and aerated as possible and just seeing if that helps?
You may consider dropping the water level and seeing if he would tolerate half the water for the length of the treatment.
You may stand a better chance of good feedback in the "Freshwater fish health and deseases" forum.