Oh that's really sad news. I'm so sorry. I would guess it's the other fish too in that case, or him growing too large to fit where he's used to hiding, like you said. If this was caused by a disease or infection, you would probably would have seen some kind of irritation, fungus, or cloudiness to his eyes before it became that severe.
You do need to get him out of there now. One of the fish could have cause the first one and gone after him again. But now that he's injured and less able to evade them, the others would be prone to pick on him too. Fish are real good at that, even if they're not cichlids.