It could be chloramine burns? Leaving water out a few days, especially with an airstone in, will evaporate chlorine, but chloramine (which is pretty common now in water supplies) sticks to the water molecules for up to 2 weeks.
Call your local council and ask them if you have that in the water - if so, that is likely to be the cause, imo.
I'm not sure if it'll heal, I have no experience with this. Grab some water conditioner asap, though..
Keep an eye on him, though, to rule out bacterial infection that can cause cloudy eye. If he gets white patches anywhere, or bloats, you could try an antibiotic.
In any case, very very clean water can only help him. Maybe add some IAL to help that healing along.