It's usually caused from an infection, injury or water conditions from what I know. Don't know if any of those fish would be bullying him, as every fish has its own personality. But he should be qt'd, and you can use antibiotics for infection and I've read that Epsom salts can be used to draw out excess fluid in the eye. But you'd have to google how much Epsom salt to use cuz I don't know that. Hope your fish gets better! Keep us up to date with his condition.