I can't be much of a help because I've never dealt with this illness in my fish, but it sounds like it could be what's called popeye? Here's an article to read to see if it sounds like what you're dealing with, and it might give you an idea where to go with treatment as well.
I'm sure there will be many people on this board who can offer more information - I don't think it's an uncommon problem!
Best of luck, hope your little guy makes a speedy recovery! :)